Ithaca started in 1880 and when bankrupt in about 1980, and went out of business a few years later. they manufacturer a wide range of double guns. their better double barre( and drillings) were made untill the end or WW-2, and are usually collectable. Price depends upon condition and the specific models. Later in the 1980s, they made a lower cost model 87 double. Ithaca also sold models they imported, including the very popular Ithaca-SKS (made by SKB in Japan), and the Ithaca -Perazzi high end singles and doubles (made by Perazzi in Italy).
So just to many models and variables to begin to guess on the price of the one you have. The low end model 87 in rough condition might sell for $400 while the engraved doubles of the 1920s might bring $20,000 in perfect shape. there are plenty of collectors of Ithaca.
There is a hardcover book about Ithaca guns and the copany, might try e-bay or Amazon or some of the bigger gun shows.
Interesting is that the name Ithaca has bee picked up by a new comany in Sandusky Ohio. they are making original designs of the Ithaca model 37 pump gun. I handled some of their guns at the NRA show earlier this year. these new products are very fine, possible better than the original model37 (better steel and machining). they have a Ithaca model 37 ultralight in 28 gauge that is superb.
I double that the 1880 was the yer of manufacture (although it could be). Most such markings are usually the year of some patent but in Ithaca I doubt it, they got into business by making shotguns on Remington designs after the Remington patents expired.