Yeah, that's my line of thinking as well.
Also... a deflection bonus (i.e. ring of protection) is worth more, mostly because it counts against touch attacks. Armor bonus (even force - bracers of armor) and shield bonus only costs half that much.
An item, that would grant a shield bonus for bonus² x 2,000 gp would be useless, since you would rather make it a deflection bonus then for the same cost, or some other obscure bonus for bonus² x 2,500 gp.
That's why I think the fact, that it would be a 'better' shield for a fighter, since it takes up no hand slot, is not relevant, even more so, if you consider the inherent bonus from the shield, which gets lost here (and if you add it on the top end of the cost scale, is worth quite a fortune effectively)!
So, all things considered, I'm extremely sure, that 16k + the 3k add-on is the right cost (remember the multiplier for multiple abilities)!
Bye
Thanee