I, too, check the gunbroker, also forum classifieds. I have a slew of forums I cruise now and again and see what folks are asking, and what folks are buying. I think it's more of a real world, real time good guesstimate. Bud's gun shop online also seems to be fairly close to going rates.
Cheaperthandirt has a online auctions too, but I'm not sure how they compare.
Just some FYI from my experience.