The ring of force shield is definitively an overpriced item. I say this because compared to other magic items that provide AC bonuses that are not an armor bonus, it cost more. For example, an Amulet of Natural Armor +2 costs 8000 gold, while the Ring of Force Shield costs 8500. They both provide a bonus to AC, they both use an item slot (it can be argued that the ring uses two, but it is unclear). Further when compared against a +1 Mithral Buckler (2020 gp), the only difference is the ring can be activated and deactivated as a free action (a useless feature if it only uses the ring slot). Both provide a +2 shield bonus to AC, but the buckler has going for it: upgraded (either by more enhancement or other special properties), and does not take up a slot for any character that does not use their off-hand for attacking. The only class that would use this item would be a monk, and even then it would be purchased after all other defensive items. As far as what the ring should cost, it should be somewhere between 4000 and 8000.
My view is that the bracers of armor are balanced around the cost of increasing the AC bonus on armors. This makes sense to me since each class is designed to have access to certain items, and abilities. The classes are balance around these restrictions. A fighter not being able to cast spells is a restriction on his capabilities, just as a wizard/monks inability to use armor. Thus by giving more armor for less cost for bracers it throws off this balance between the classes, since it would allow the non-armored classes a faster AC progression. Saying that it is ok to have this imbalance in items since class X is gimpy does not justify an imbalance in items. If class X is gimpy then change class X not an item that is usable by anybody. Even if you can only forsee class X using the item at the moment, there is no telling what the future will hold, and I personally prefer to use rule 0 as evenly and rarely as possible. (since if it becomes broken in the future you have to reverse course on the ruling)