D&D armor prices are balanced against each other, not a precise equation
The overall pattern to the armor prices in PHB p.145 is that no one armor is universally better. Any armor that is better in all practical respects than another is offset by a significant price difference.
For example, chain shirt and scale mail both cost 50 gp, because scale mail gives 1 point higher AC but has disadvantage on Stealth, so there's still a reason to use either. But breastplate is 1 higher AC than chain shirt without the disadvantage on Stealth, and costs 400 gp as a result.
An armor that renders another completely useless should, in general, be accompanied by a significant price rise. This protects each armor as a valid choice, at least until the point where you can afford the best in class. There's a valid reason to buy any given armor.
In the earlier D&D third edition, the armor list lacked this property. For example, practically nobody wore chainmail because breastplate had the same AC, was superior in every way, and and cost only 50 gp more.
Another effect of the item prices is to drive the game. For example, pricing full plate at 1,500 gp puts it out of reach of first-level characters, prevents them from being unhittable early on, and gives them a motivation to save their money. At the same time, there are cheap armors in every category, making heavy armor a valid option from first level; compare this to 3e, where fighters usually started with medium scale mail for price reasons.
In short, an armor is too cheap if it completely obsoletes any other armor in its price range, and too expensive if it is completely obsoleted by another armor. Within that range of values, you are free to set the price as makes sense to you.