I assume you're referring to the full size model? While stainless models can sometimes command a slight premium, it's not really that extravagant, especially with the standard USP's, and it's also hit or miss.
That's a fairly large window, but I'd estimate you're looking at anywhere between 550-900. There's a possibility it could go lower for the heavily used (as well as higher for the NIB), but that would be contingent on a number of factors. Finish wear/damage on the slide and/or frame, included accessories, round count, mags, LE trade-ins, etc.