I'd charge out the wazoo for an item of an uncommon stacking type, because each additional bonus means more than the previous one.
Here's what I mean. If your only defense is bracers of armor, everything will hit you. If you add a ring of prorection, weak monsters will sart missing you. Add an amulet of natural armor on top of that (bringing your AC into the 20s), and the big monsters may actually miss part of the time. The amulet costs twice as much as the other two bonus types, because the third stacking type is what gives you the greatest benefit.
So for the AC bonus alone, I'd charge 64k (four times as much as equivalent bracers, or twice as much as equivalent natural armor).
The bonus to hit should also be expensive. A +4 weapon would cost you 32k. From one point of view this item is less useful than that, since it doesn't increase your damage or let you penetrate DR; on the other hand, it does stack with your existing weapon bonus, which is a huge benefit. I'd probably jack the price up a bit, but for now let's be conservative and leave it at 32k.
Since it's a slotted item, the secondary ability costs double.
Total market price: 128k.