The easiest way is to look at the gun auction sites on the Internet and see what similar models are going for.
http://www.gunbroker.com/Shotguns/BI.aspx
http://www.gunbroker.com/item/657222934
Rough guess for a run of the mill 100 in like new condition? About a thousand bucks from the above sources.
There were some special models made, but you would need to research that. Much depends if it is an early Ithaca or the more common version made in Japan, later on.
Thanks for the sources, I’ll check them out.......