Without taking a side on Wis +1 AC vs just +1 AC initially, I will say it's worth noting the AC gained stacks with dang near everything. Stacking defense is a big deal. It's normally a no-no (items don't give dodge bonueses for this reason). So you've got to figure that if an ioun stone that gives +1 insight bonus to AC is worth 5k, then an item that grants an unnamed bonus, which stacks with everything, muts be worth more.
And yes, ioun stones don't take up a slot. However, even if you apply the x2 multiple, which sems to be consistent with the ability boost stone prices, that's a base cost of 2,500 for +2 AC of an odd bonus type. For no bonus type? I can easily see +1 AC of no type being 10k, and the unarmed damage another 3 k. And since drawbacks to items don't seem to be worth more than a 30% reduction, I'm not convinced the fact you can't wear armor is relevent (it still stack with armor bonuses from other sources).
Let's put it another way. If we assume you price the item with the idea that whoever puts it on has just a +1 Wis mod (if you have no mod, why wear it?), it's worth +2 AC if the Wis adds. For a standard item, that would be 8,000 gp. But we know an unusual bonus type is worth x2.5 (see the ioun stone, above), so that should be at -least- 20,000 gp. An that's assuming being of no bonus type isn't worth even more.
It seems to me the math supports a flat +1 AC, given the nature of the bonus.