It's only worth about $150...which I will send to you in a money order, TOMORROW, if you want to sell it!
Seriously, it's worth whatever someone will give you for it. Do a search for "comps". In the real estate business, that means looking at similar houses that have sold, in your area, recently. Gunbroker.com is probably the best place to do this kind of research. A lot of times, the "buy now" price for a given gun is about the most you should expect to get for yours and if you knock off 5-10%, that's probably more realistic.