I’ve been working a tad more on my huge table of prices and stuff.
Then I thought… if the king of France was playing D&D 5e in 1385, how many plate armors could he buy? As it turns out, using the entirety of the kingdom of France’s capital (including all taxes etc) for a year one could buy 2700 sets of plate armor.
A knight banneret (an “officer” or higher ranking knight) would need almost two full years of pay to buy a set, using the 5e price of 1500 gp. (The higher figure I found had them making around 860 gp per year… in full year-long service, I think?).
Theoretically, one gp in 1385 corresponds to around $3500 today. So the cost of plate armor in 5e would correspond to maybe 5 million USD. That seems quite a bit too much, compared to the 3-18 “gp” (livre / pound) that’s in the sources I’ve found.