Depending on the condition of the pistol, $650 for a HK USP45 with a Surefire sounds like a fair price. If it is a police trade-in, then I think that asking price is a bit much. Without knowing what type of Surefire is on the pistol, it's hard to estimate its value.
The last used USP45 (with standard finish) I purchased cost me $500 plus shipping and transfer fee. That USP was made during the first year of production and was in like new condition. The last two used stainless steel USP45s I purchased cost me $650 and $750. They were both in excellent condition. I purchased all three of those USPs within the last six months.
USP45s do not really have any weak points. The firing pin design was updated in the early 2000s, but I wouldn't let the old design keep you from buying this pistol. USPs should come with either a grey or black plastic case, two magazines, and the owner's manual.
I'm a bit of a HK USP45 fan, so I would recommend picking it up.