You could go to Gunbroker.com and see what the asking prices are on similar guns are.Just keep in mind, "asking price", and what the gun actually sells for may be two different things..Look at what folks are actually bidding. This should give you a close figure to go by.
Remember condition is often the determining factor when it comes to value, especially on older guns...Your shotgun is probably worth between $250 and $500...$250 if it's scratched up..$500 if it is in pristine condition,