I don't have any problem making it costly because it should be costly.
Actually, I don't have a problem with the idea that you can buy a piece of armor for 1'000 pieces of gold. if that is their cost, then very few people will have them, but we are all going to assume that knights still wear armour in battle, just that their armour is not nice enough to be specifically noted on the family page (or wherever it will be noted), just like it's ok to assume everyone has a horse if they want to RP they have a horse without having to buy one, or that it's ok to assume they have a sword even though they don't have the Swordy Sword of Swording.
I dislike, however, with the idea of the game-mechanic 20-gold armour. It adds nitty-gritty detail to the game that is actually detrimental to its nature.
