You should be able to find your gun listed in The Blue Book of Gun Values, The Standard Catalog of Firearms, or Gun Trader's Guide. Your local library should have a copy, or you can just browse through one at Barnes & Noble.
I imagine the value of your shotgun would depend entirely on an independent appraisal to determine NRA grade. Look near the serial number stamping on the barrel-side surface of the forearm - is there an "F" stamped nearby? If so, I believe you have an Ithaca Flues-model double like mine, manufactured around 1910 and originally selling for about $37 (1910 dollars). I've never had mine appraised but would not sell it anyway, just because it's a hoot to shoot and stays tight with their automatic play-in-the-action-takeup thingamabob. It wouldn't be worth quite as much as the Ithaca New Model doubles that followed the Flues but should be in the ballpark, and there are a lot more New Models out there for reference. As long as you have the serial number, I should mention that information on individual guns was available from Ithaca, but that was back in 1971. Good luck, and if you keep it, good hunting!
Here's something I just found that might help you - this gun may be similar to yours:
http://www.dixiegunworks.com/product_info.php?cPath=21_24_66&products_id=622