I'm wondering would the cost of the 100 Years War to the French or English be relatively similar to the cost to those countries centuries later in the Napoleonic Wars or would it be far cheaper or more expensive? How would those costs compare to World War One and Two? A more modern war like the Falklands or the occupation of Iraq?
In short, I'm wondering how the cost to the leadership or government of a country going to war, particularly major conflicts with another major power, has changed with technological and social changes.
We're going to paint a broad picture here, because "medieval" encompasses a huge period of time, but the TL;DR is this: It's pretty much impossible to relate costs now to costs then.
This is more difficult to calculate than you might think. In our modern day market economy we tend to compare and contrast how things are today with how they were, say, a generation ago. So, nowadays we might pay $3.50 per gallon of gas, but fuel only eats up (say) 1.5% of our total budget, whereas in the 1970s $1 gas was 6% of your total budget (I'm making these numbers up, but you get the idea). We get (sort of) an apples to apples comparison, in large part because we have similar priorities and also because we have a monetary system in which things like inflation and relative costs can be tracked. In the medieval era this really wasn't possible.
Let's take a look at some stuff from the early modern era that might help give some perspective. It's not medieval, but it gives you a picture of how different things were. For example, a peasant woman would often work until her mid-twenties before getting married. What was she doing? She was saving money for a cow! See, that cow represented a source of livelihood for her and security in marriage. The cow produced calfs, which were useful for eatin' or making more cows (or money from selling them), and it produced milk, and when the milk and calving are over, she's still edible. Okay, so she paid money for the cow, right? She could sell milk or calves, right? How is that different from the modern day? Well, for one thing, she paid the customary price for that cow. In other words, price wasn't set by the market, it was set by custom. This could be monkeyed with a little, but in general trade was local and social pressures kept prices static. As you might imagine, when a failed harvest made food scarce and farmers inevitably raised their prices, people got upset. They often rioted. Sometimes the riot took the form of folks breaking into a storehouse and distributing food, other times it was more violent. Landholding worked differently too. Peasants would work land that they "rented" from the local lord. Their rent, though, was a proportion of the crop and social deference. In return the lord provided them a place to live, a place to bake bread (he usually had the ovens), he would even represent his peasants in court. So, no money changes hands at all. Instead, this was all about deference and power. It was a cooperative system in which money mattered little and when it was used, prices were set by custom.
If we go further back we actually find less money in circulation, which means that social obligations become even more important. The medieval era often saw Europeans working within a tightly knit web. They produced armor, not because they were paid in coin (though sometimes they were), but because someone provided something for them. The thing is, whatever was provided may have been ephemeral in nature or purely spiritual. Or, trade may have happened, but it would have taken the form of barter. It's all very difficult for us to imagine, but it's how things worked.