I picked up a barely used 4" GP100 for $300 a couple months ago.

check out galleryofguns.com use the gun genie search it will give you price quotes for up to 100 of the nearest dealers within any number of miles. if look through enough dealers you should be able to find one in the 480 to 500 range pretty easily. goodluck
I just paid $400 for a slightly used 4" stainless at a show.
I paid $200.00 for my used 4" GP100 blued at a gun show about 2 years ago.