The armor's elemental resistance is something I based off of the elemental armors in Final Fantasy IV (fire armor protects you against ice, ice armor protects you against fire), although I do suppose that your suggestion is more sensible. I'll have a go at switching that around.
And for the armor prices, yes. You get a price increase based on the size, and another based on the type of crystal. For example, getting divine crystal full plate would cost a total of 7,000 gil (1,500 gil for the full plate, +1,500 gil for it being heavy crystal armor, and +4,000 gil for being divine crystal). It's rather expensive, but if you ask me, something definitely worth considering, especially if you want to go all-out and get cosmic crystal armor.
And for the armor prices, yes. You get a price increase based on the size, and another based on the type of crystal. For example, getting divine crystal full plate would cost a total of 7,000 gil (1,500 gil for the full plate, +1,500 gil for it being heavy crystal armor, and +4,000 gil for being divine crystal). It's rather expensive, but if you ask me, something definitely worth considering, especially if you want to go all-out and get cosmic crystal armor.