A good way to get an idea of how much a used firearm cost is to check out the going asking prices on a website like:
There is a post on this forum in the General Revovler area (I think) on how to judge a used revolver. It may be sticky.
One thing about a Ruger GP100 is its deserved reputation for durability. I've heard of gun ranges that rent guns, having a GP with an excess of 10,000 rounds through it and its still working. If you come across a ragged GP100 keep in mind what someone had to do to make it like that.