Slot wrists; Price 1,000 gp (+1), 4,000 gp (+2), 9,000 gp (+3), 16,000 gp (+4), 25,000 gp (+5), 36,000 gp (+6), 49,000 gp (+7), 64,000 gp (+8)
Why would anyone spend that kind of money when a scroll of mage armor is 25 gp (or 12 gp 5 silver if you craft it yourself) and lasts an hour? Or even better, with the Craft Wondrous Items feat, you could create a robe of mage armor for (best estimate) somewhere around 900 gp (base price 1,800 gp) or even less if you limited it to a certain number of uses/day. That would have the added bonus of going into the body slot, which most wizards/sorcerers never use anyway, freeing up the wrists slot for something potentially more interesting.
I cannot think of an in-game reason why such a magic item doesn't exist. +4 armor, no arcane casting penalty, no armor check penalty, negligible weight? Yes please! With a market cost of 1,800 gp, who in their right mind would ever wear a chain shirt again? Or pay 4,000 gp for bracers of armor +2?
Of course, I can think of a perfectly good out-of-game reason: namely that it's totally broken! 'cause with a market cost of 1,800 gp, who in their right mind would ever wear a chain shirt again? Or pay 4,000 gp for bracers of armor +2?
These are the kinds of issues that reduce game designers to gibbering husks, late at night.