I can't filter through all the search results which are actually related to gaming. However I know one recent documentary about Musashi where they briefly mention armor mastery and training was in "Samurai" on the history channel with Mark Dacascos. However, that is one such source I can note. There is more to wearing armor than just putting it on like clothes though.
I'll need to see the proof, but I know where to look now. Tell me, from what you saw of the documentary, does mastering the wearing of leather boots take as much skill as mastering a Wakizashi? Does it take as much time to learn to wear a briastplate as it does to kill someone with a throwing knife? Does wearing a steel pauldron take as much effort to master as sneaking into a bank would? You know that you can become a master of light armor in Oblivion by wearing nothing but a leather hat for awhile.
In the middle ages labor was cheap. Armor cost so much because steel was hard to procure, because of low supply and high demand. It was mostly from the cost of material that armor was so expensive, because so much metal was needed. If in the middle ages armor was more easy to provide (via magic) and people didn't want it as much (Ebony and Glass are better) then in the middle ages STEEL armor would've been cheaper. However, daedric, ebony, and glass armor would cost a LOT because people would want them so much.
I'm not postulating about the Elderscroll's economy, I'm noting on the economy of the middle ages and how the availability of armor material alternatives present would likely have driven down the cost of STEEL armor. The reason the devs have the value of armor set at what it is, is a decision entirely all their own to make. I'm just providing a reasonable explanation.
Additionally, there's no reason that armor shouldn't be within the price range of a peasant. Most peasants in TES could probably afford a suit of 150 gold armor, but that would be like a person now a day buying a suit of armor about. Considering the cost of other in game items it'd be worth about two-four weeks of food, just like a suit of shoddy armor might cost online in real life, yet how many people do you know who own a suit of armor? Most people would rather eat.
Labor was cheap, but that doesn't mean skilled labor was easy to come by. That's why the guild system got started; to train people in the specific trades, to make a monopoly. The steel was harder to procure, but it's not like they had a shortage of it in the west. In the East, they definitely did. That's why the Japanese didn't have steel plate armor, they couldn't get enough steel to use it practically that way. As far as demand goes, not a lot of smiths should be making steel armor anyway. Smiths are mostly needed to make nails and horseshoes, and cookware. It's also worth pointing out that the higher quality steel you had, the thinner and lighter the armor was, so better technique allowed them a cheaper overhead.
It's a dangerous thing if armor or swords are in the price range of a peasant. The empire in theory should keep the production of armor low by squeezing the guilds, and the cost high to prevent a group from arming itself on a whim. Yes, the peasant might be hungrier for a week if they donate their lunch money to a cause, but that cause can roll out soldiers armed with decent weapons and armor rather quickly if they get some membership. The nobles have to do what they can to prevent peasant uprisings. Love is not the answer. The cost of the plate armor should be more like the cost of a new SUV in modern terms, paid in full up front.