gabriel_z, the issue is that there is an existing item that already uses the spell Mage Armor as part of its creation. The equations as before mentioned are guidelines and you should first compare a new item against existing items.
Also mage armor is better compare against a chain shirt, since both do not prevent moving at full speed. But you are not simple playing gaining the same AC bonus, there are a number of other benefits from the item. As has already been mentioned, you gain the AC against incorporeal attacks, but further you also do not have to deal with armor check or spell failure. The cost that Bracers of Armor represent is the cost of increasing ones AC through enhancement bonuses over the base armor of the object. This is why Bracers of Armor has the cost equation as the cost to increase the enhancement bonus on armors. The difference is that the bracers assume a +0 base armor.
Also mage armor is better compare against a chain shirt, since both do not prevent moving at full speed. But you are not simple playing gaining the same AC bonus, there are a number of other benefits from the item. As has already been mentioned, you gain the AC against incorporeal attacks, but further you also do not have to deal with armor check or spell failure. The cost that Bracers of Armor represent is the cost of increasing ones AC through enhancement bonuses over the base armor of the object. This is why Bracers of Armor has the cost equation as the cost to increase the enhancement bonus on armors. The difference is that the bracers assume a +0 base armor.