Thank you for specifying such a specific time frame! I will make my answer even more specific and focus on England, where the Wars of the Roses lasted for just about the time frame that you specified.

First, some social history. In England in the 15th century, those who fought in full armour were by definition 'men at arms' - even if they weren't titled knights (they very often weren't!) they fought like knights. These were the lowest echelon of the military elite, but they were still had some pretenses to being elite - they are often styled 'esquires' and referred to as 'gentlemen'. On the other hand, there was some fluidity between the upper reaches of the peasantry and these minor military aristocrats, or with the professions (like lawyers, who are increasingly important in society). Famously, the Paston family started as ploughman, became Lawyers, and then knights and courtiers, all within a few generations. In addition, the younger brothers of men at arms might serve as archers. So I will interpret your question as pertaining to these untitled 'men at arms.'

Prices for full harnesses of plate armour ('suits' of armour as we would say today) varied greatly. This depended on a number of factors:

• Whether an armour was 'made to measure' or bought 'off the rack' (or even used). While armour does need to fit its wearer, armours were indeed pre-made to general sizes and then presumably adjusted after purchase (re-strapped, perhaps the height of a greave would be adjusted, etc). These kinds of 'mid-range' full armours (not as rough as the munition armours of the later 15th century and 16th century, but not really fine quality either) were much of the business of the city of Milan, perhaps the greatest armouring center in Europe, as well as centers like Nurnberg and Koln in Germany. In England in the 15th century, many armours for untitled men at arms and 'esquires' were made in Italy and sold, sometimes in large numbers (some lords ordered as many as 100 harnesses at a time during the Wars of the Roses). These would not be made to fit. By contrast, the titled elite seem to have often worn natively produced English armour, made to fit. Some others wore fine quality imported armours. They paid quite a bit for these 'bespoke' armours, just as people pay a lot today for a bespoke suit.

• The reputation of the armourer and the place it was manufactured (and what sort of quality control, if any, that the armour was subjected to - in places like Nurnberg inspected armour bearing city marks sells for more than unmarked armour). As one example, an order of 100 complete armours made by Antonio Missaglia (of the most famous family of armourer-capitalists in Milan) cost 200 lire apiece (for a total of 20,000 lire - the Missaglia's business was fabulously profitable), while 12 armours made by the less distinguished armourer Pier Innocenzo around the same time cost 32 lire each - around 1/7 the price.

• What kind of finish and decoration were applied to the armour. This is mostly relevant to the armours of lords and princes, and in some cases in the 16th century the cost of the gold alone for gilded armours cost more than several 'ordinary' harnesses of knightly quality. Though not as extravagant as the armour of kings, the decoration of the armour of lords and knights was more extravagant than that of untitled men at arms, and the difference is reflected in the cost.

So, costs. On the low end ready made armours could be had for fairly modest sums - amounts that still amounted to several months wages! One of the lower amounts I have seen is the 5 marks (a bit over 3 pounds sterling) that Lord John Howard paid for an armour that was complete except for a helmet and gauntlets (presumably, this was an armour for his retainers, not himself!). The average cost for the large order of 100 armours mentioned above is a bit less than this. On the other hand, John Cressy paid between a bit over 8 pounds and a bit over 6 pounds for harnesses for himself and his esquires (and this was also from Milan), as u/Dashukta mentions. This is a good figure for the 'mid range' of armour - good quality, but not made to measure. Finally, truly custom armours from the London armourers could be as much as 20 pounds or more. Fine quality, custom armours from abroad (generally Milan or the Low Countries) would be similarly expensive.

So, what do these numbers mean?

In the early 15th century an archer in the English army made 6 pence, which is to say, half a shilling or 1/40 of a pound. This is similar to the wages of skilled (journeyman) artisans. So an armour that cost a bit over 3 pounds would cost more than 120 days wages for an archer. Men at arms made a full shilling, so the armour mentioned above would cost a bit more than 60 days wages for them. However, often armours would be bought by the man at arm's contracted employer (someone like Lord Howard), so this cost might not be born by the individual man at arms. On the other hand, a 20 pound custom harness would cost the equivalent of over one year's wages for a man at arms and 2 years wages for an archer.

You may be interested in my previous answer about Manufacturing and Supplying the Weapons and Armour of a Late Medieval Army.

Sources:

-Alan Williams, The Knight and the Blast Furnace

-Tobias Capwell, Armour of the English Knight 1400-1450

-Matthias Pfaffenbichler, Armourers

Well, the cost is going to depend on how wealthy the person buying the armor is.

Asking how much armor costs is sort of like asking how much a car costs today. It totally depends on how fancy the car, and for what purpose. Just as commercially-available cars today range anywhere from a rather affordable Toyota or Chevy economy model to an exorbitant Koenigsegg or Lamborghini supercar, plate armor ranged from mass-produced "munitions" armor (although, that was more a 16th century thing) to intricately decorated and guilded tournament armor made for kings.

But, there is a period reference from 1441 in England, just prior to the time-frame you asked for, for the accounts of one Sir John Cressy, who purchased a Milanese-made harness "ready-made" for £8 6s 8d (8 pounds, 6 shillings, and 8 pence) and armor for a squire between £5 and ~£6 1/2. At the time, a man-at-arms on campaign could expect a wage of about 12 pence per day (in other words, it was worth about half a year's wages).