Pixels are, well, pixels. It makes no difference whether you’re covered head to toe in plate armor or just wearing a g-banger. A costume is a costume and it costs whatever TF zos wants it to cost - obviously based on saleability.
Every single product or service on earth is priced based on "saleability" (demand and supply).
Costumes are not only "pixels" - that's just the way they're used - , there's artists', coders', designers' and testers' wages behind them.
That being said, it's true that those costs are probably the same if it's a complete armor or a bikini. And it's true that those threads are quite ridiculous ;-)