There are no doubt occasional deals on USPs that are used and I’m sure you got yours for a great price, but it’s not something that’s a given.
Prior to the Coronapocalypse and the associated gun-buying spree, quite a few nice condition USP45s could be found selling in the $425-$475 price range on Gunbroker. I know, because I was following prices at that time. The example I purchased for $450 from a LGS in November of 2019 was no fluke.
USP45C, USP45 Tactical and USP9 examples sell for more money, no doubt. But standard USP45 and (especially) USP40 examples carry less of a premium.