Not to be a smart ass, but a gun is worth only what someone is willing to pay for it. The scope of those wishing to pay top dollar for a 50th USP is pretty small, probably less than 50 people in the US, IMO.
What you will find is folks willing to pay just a hair over a standard USP price for one in the same shape, size & caliber. And when I say "over", I mean no more than 10% over.
Commemoratives rarely, rarely go up in value. Most barely hold their value for a non-commemorative model w/o all the bling.
As for what it may be worth, take your purchase price & add 25% to start with on selling price. You can always come down.
Hopefully Big Bore will be along to add his .o2 to this as he's pretty much our resident guru.
My advice is worth exactly what you paid for it..........