I think multiplying the magical effect cost by 1.5x because the item has a purely mundane function is pretty overboard. If its later improved to a +10 bonus, do you think that should also be subject to the 1.5x ? In either case, whats to stop the subject wearing a magic belt of +x skill bonus, and a mundane belt to hold his items ?
Now granted you could say that waist slot isn't an affinity for +skill bonuses to use magic devices, but in terms of the item it makes sense so I'd probably overlook it in this case. Perhaps 2800gp or so; a high quality mundane item (300gp) with a magical effect enchanted on to it (2500gp).
