Soldiers are ALWAYS paid. Maybe not always a salary, but they are typically not in it for that anyway.

A spearman, for example, was paid about 2p per day around the time of the battle of Crecy. [source: The Road to Crécy: The English Invasion of France, 1346]

Archers a bit more, and anyone with a horse or better armor caused those wages to start escalating quickly.

As for how it was paid for? Looting, taxes, income from lands. And loans. ALL THE LOANS. Many of which were defaulted on, causing whole banking families to fail.

The King of England, Edward III, rather famously, asked the House of Commons for a 300,000 pound grant to pay for his invasion in the summer of 1339. They eventually agreed in return for some concessions in 1340. [source: The Encyclopedia of the Hundred Years War] This actually delays the start of the war somewhat, but doesn't really have much of an impact on that phase of the war.

Although this led to some early successes, Edward essentially went bankrupt financing this endeavor and lost most of Scotland in the process.

The Peruzzi's and the Bardi's were two of the largest banking companies in Europe at the time. Emphasis on WERE. According to the contemporary chronicler Givanni Villani, they went bankrupt loaning money to England for this war. [source: A History of Business in Medieval Europe, 1200–1550]

Now this is PROBABLY not entirely accurate, but it does go to show that the English crown was borrowing a LOT of money and not paying it back.

This, along with the French crown having similar financial difficulties, led to the Truce of Espléchin in 1340, which effectively ended the first part of the war.

Edward then raised taxes and manipulated the wool trade for a few years to get together enough money to pay his debts down enough to try again in 1345. This went fairly well for him, leading to a victory at Crecy in 1346, but the black death arrives in England in 1348 and puts things on hold for a bit.

Meanwhile, Edwards son, also Edward, this one called The Black Prince, comes of age and decides to restart the war in 1355. By this point the plague had mostly receded in England.

This leads to another English victory at Poitiers in 1356, leading to the capture and holding for ransom of John II. His ransom was set at 4 MILLION ecus. I don't know what that is in pounds, but it was enough that the French government to default on the payments and have to make arrangements as if England was a debt collections company. Bits of land such as THE ENTIRE ANGEVIN EMPIRE were used, along with hostages from every town in France, as collateral.

Disagreements over this and other matters lead to Edward invading and just about taking Paris in 1360, but then Edward lost the battle against the weather. A freak storm wiped out a good chunk of his army, and, exhausted and broke, Edward and Charles the V were able to work out a treaty that was eventually ratified by Edward and John as the Treaty of Calais in October of 1360.

Meanwhile, remember that Black Prince guy? Yeah, he is now, and has been, the Heir of Edward the King, so of course he is spending daddies money like it is going out of style. He spent an ABSURD amount on getting his buddy Peter the Cruel back on the throne in Castile. And did they pay him back? Of course not.

So how does Edward handle this? He raises taxes on the province of Aquitaine for years. And when he then ignores the peoples complaints about it, they turn to the French crown and ask Charles for help. He, starting in 1369, jumps in and starts taking back all the bits of land that were taken in the Treaty in 1360.

This is an important part of the war, but not relevant to your question. Taxes, loot, and loans. I hope this demonstrates how and why that is the answer and how money was a HUGE motivation for the start and continuation of the war.

Hello ! I'll try to add some French perspective and point of view to what my excellent English-specialist colleague said.

I do think the distinction between France and England matters on that subject, mainly because they were, in fact, very different in their respective wealth, organization and armies.

I'd also add that French have a slightly different point of view about the war, so it seems, since we consider the first part of the war to end around the accession of Henry V to the throne, tending to consider the reigns of the Plantagenêts as one part of the war, and the rise of the Lancastre as the second one. But that does not matter much for what is at stake here.

To put it short : France was quite richer than England in the 1330's, and could rely on a much larger population. Edouard Perroy estimated the populations of France and England at about 15 millions and 5 millions respectively, in 1328^(1).

Their respective organization were, also, quite different, and particularly when it comes to military organization. Both kings relied on the feudal "ost", the assembly of knights and lords summoned to fight for the king, but that's nearly the only common point. English soldiers, as u/Talothyn wrote, were all paid. That is not necessarily the case in France. Theoretically, the noblemen owe a 40 days service for free to the king, every year. That is extended to 3 monthes for communal militias. That is, though, theoretical. In reality, lords were bringing to the ost forces way inferior to what they could muster for their own private conflicts. Moreover, one has to consider the time needed to assemble such a force, and weeks (or sometimes months) could pass between the king's call and the actual arrival of troops to the gathering point. Let's not forget that you indeed needed weeks to go from Montpellier, for instance, to Normandy, Picardy, Brittany, were battles were typically fought.

This lead the French army to be rather small (considering our modern standard, obviously), but the kings of France were still generally regarded as the most powerful princes of Christendom (which was relatively true).

One important thing to note is that France was quite richer than England : fertile lands, renowned wine, flourishing economy in general, with 15 million people to pay taxes and create riches. On the other hand, England wasn't so. Its nobility, in particular, could not really rival the French one when it came to money, lands and income. That is one factor of the decline of English chivalry and heavy cavalry in favor of longbowmen and footmen : equipment was cheaper (not the only reason though, of course).

The discrepancy was not leveled with the ravages on French lands during the war. Until the end, southern France in particular remained nearly intact, and rich, and its continuous allegiance was one of the reasons why the Dauphin, Charles VII, was able to resist and finally win (I'm talking about southeastern France, southwestern being English or disputed during a great part of the war).

Now, let's talk about how they were paid. As u/Talothyn said, English armies were paid and the English crown (and soldiers) relied heavily on plundering and ransoming to fill the coffers. But where did the French king got his money from ?

Well, first, from taxation. I mean by that indirect taxation. I wrote an comment here that explains the different ways French kings got their money https://www.reddit.com/r/AskHistorians/comments/d8kdr0/how_absolute_was_the_reign_of_kings_during_the/ . The answer's examples were essentially about Burgundy, but the king of France could rely on basically the same income sources.

Now what I would like to add to what I previously said is that the Hundred Years War was a MAJOR turning point in France centralization and administration. Using the pretext of disasters (the capture and ransoming of king Jean II le Bon, notably), Charles V of France was able to implement the first "permanent" taxes, that greatly improved their resources (after the defeat of Poitier and capture of his father Jean II, in 1356). If in theory those new taxes were supposed to be temporary, the continuation of the war, the ravages of the Great Companies, the payment of Jean le Bon's ransom, made them nearly permanent. To give you an idea, an administration was created to manage those "extraordinary" ressources. And believe me, when the French create an administration, there is nothing temporary to be expected (in fact they were abolished at Charles V's death in 1380, but only a part of them, and many were in fact re-launched years later).

France also used loans, of course, with the great advantages of having rich lands and great incomes to back it up. One can lend more easily when he knows that his payment will be backed by the riches of the kingdom of France. Although that lead to financial difficulties, as that was pointed above, France did not need to rely as much on loans to keep on fighting, mainly because of its bigger "resources pool". One thing is important : although French monarchy was broke on many occasions during the war, France (meaning French population) wasn't (although it was significantly poorer).

One explanation to the end of the war was the lack of English will to go on fighting, partially because the French wars were not sending money to England anymore. I developed a bit the idea in another comment (sorry for self-advertisement, this is only in order to avoid to rewrite the same thing and fasten the delivery of this answer) https://www.reddit.com/r/AskHistorians/comments/defaaj/in_medieval_europe_did_the_average_peasant_care/. To give a quick idea : war in France was an amazing source of income : pillage, ransoms, ... With the beginning of defeats, ransoms going the other way around and more and more fortified and garrisoned cities becoming invulnerable to "chevauchées^(2)", money wasn't flowing anymore, leading to war exhaustion and lack of fighting spirit in the English population (again, not the only reason, but you've got the general idea).

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^(1) PERROY, Edouard, La Guerre de Cent Ans, Paris, 1976 (1st ed. 1945.)

^(2) A chevauchée (from French, literally, chevaucher : to ride a horse, is a war operation typical of English in early Hundred Years War. The main idea is to gather a rather small army, but of good quality, and to make a devastating raid in ill-protected lands (countryside, small cities, ...) before retreating to friendly places before the great French ost can gather and catch up. The idea is to avoid big land battle, that are hazardous at best, especially against such a powerful and renowned army as the French one is at the beginning of the war.

For sources, alas in French but I do not doubt that you can ask for English sources as well on that question, although they might be focused on England, just like French sources tend to focus on France :

AUTRAND, Françoise, Charles V : le Sage, Paris, Fayard, 1994.

BOVE, Boris, Le temps de la guerre de Cent ans : 1328-1453, Paris, Belin, coll. « Histoire de France », 2009.

PECOUT, Thierry, Charles V donne naissance au franc, Historia n°107, may-june 2007.

SCHNERB, Bertrand, L'État bourguignon : 1363-1477, Paris, Perrin, 1999, (focusing on Burgundy, but it will give you a good example of financial relationships between nobles, peasant, bourgeoisie, ... at the time).

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I could provide more on specific subject if you need (and can read French).

Hope that helps !