Gunbroker is a good resource. You can browse current for sale and sold guns to get a good idea. You can also get an offer from cash for guns to give a solid start point.
Go to your nearest gun shop that takes trade ins and see what they will offer you on trade. Add 20%-25% for their margin and that should give you a close used market price.
I also have a gun show right down the road coming this weekend. Might take a walk through there and see what is going for what. Be worth the entry fees.