For manufacturing of new goods, flip it upside down. Nobody will make armor that requires super-expensive materials and skills if it gives less protection than something cheaper and easier to make.Alright, stupid question time, but: shouldn't the armor's pricing be dependent on the cost of materials and labor involved and not the AC it grants?
For historical goods, the price of the armor as a tool is going to be based on how effective it is.
Armor may also have non-tool use value.
That means that, if you want non-variable magic item prices, armor should be priced based on how effective it is. You could note that some armor costs far more to manufacture, and the only armor you can find in that type is from ancient civilizations who couldn't do better.
Now we can check that the resulting prices don't result in something stupid happening. Ie, we don't want half plate to be 3x as expensive as full plate, or the like.
But once we have a price system that is based on an ordering of the utility of the item and about when a PC should be getting the item, we can then see if we can write justification (in terms of cost of materials and labor).
Or we can just throw together some random economic simulation and derive prices from it. The most likely result will be something like "nobody makes anything besides mithral plate, all other armor is worthless".
In the real world, that happens all of the time. For about 100 years, the internal combustion engine car dominated all other kinds of personal transportation in the industrial world. There wasn't a fun mix of Steam, Electrical, ICE, and Nuclear powered cars that had trade offs.
Writing out stats and prices for 1970s Steam, Electrical and Nuclear powered cars would be a waste of time in a game. If you presume some economic system that isn't taylored to the world you are trying to describe, where people wear those varieties of armor, then you'll almost always end up with "well, choice X is best, everything else is garbage", because that is what happens when you build that kind of system without the feedback loop of "but, I want there to be a reason to wear medium armor".
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