I can't prove this, but I have come to the conclusion that USP's lose value quicker than many other guns. In looking at both new and used prices over the past couple of years, it appears to me that they sell new in the $700 to $800 range now, but used in the $475 to $575 range. I bought one used myself over a year ago for $550 (.45 full size, stainless slide), and it was in almost-new condition.
I'm not sure why this is, but my theories would be that some people buy new USP's and then decide they don't like them after 100 or 200 rounds, and want to sell them to buy something else - or maybe the new ones are a little "overpriced" compared to competing guns like Glocks or Walther P99's, and the used ones quickly fall to parity with these competitors.
At any rate, if I wanted another USP I would definitely look for a used one rather than a new one, to take advantage of this "aberration" in the market.