In all honesty, asking a gun blogger the value of a specific firearm is probably not a good idea because there are far too many variables involved, the condition of the firearm being foremost.
The best thing you can use for estimating a gun’s value isn’t free, it’s S.P. Fjestad’s Blue Book of Gun Values (sorry, but my copy is out of date). If you don’t want to spend money to get an idea on the value of a specific gun, stop in at Gunbroker.com or GunsAmerica.com to see the settlement prices for similar guns. If your firearm is an antique, a book from Flayderman.com may be helpful (sorry, but my copy is out of date).
EDIT 2/5/2012: CLICK HERE for Gun Values Board