You can google a firearm and see some sales that have taken place recently. There are several books on gun prices, but Fjestad's is probably the best. They have some stuff on-line, but mostly they're trying to get you to buy the book or DVD.
You can also ask on here and someone who has a copy can look it up.
I've found that looking up auction sites and finding out what they sold for or listed for, is a good guide. Remember, a gun, like any object is only worth what someone will pay for it and what the market bears at the time you're selling it.