Generally, the real value of anything is what someone is willing to pay for it. With that said, I did some research (you're welcome).
Using Gunbroker.com
https://www.gunbroker.com/ I searched for 1897 Winchester pump shotgun.
I found 31 items for sale. Of that 31 items I only chose the 12 gauge models and of those threw out the desirable models such as the take down, trench guns etc. I included guns that were for sale, at current bid, and starting bid as of today. I also included the guns for sale/auction that I felt were priced too high to be taken seriously (2). There ended up being 13 guns that met the criteria.
Drum roll please.... the average was $632.
I have been a SASS shooter since 2002 and own a couple of 1897s ( 2 from 1904, 1906) and have witnessed or been involved in conversations regarding buying, selling, looking for 1897s since then. The $600 figure, at least in that group, is fairly accurate. And that sport is still fairly well attended and bringing in new shooters all the time. The 1897 Winchester, or replicas, is the only pump shotgun allowed in that sport. So advertising on the SASS site might be advantageous. Mainly because that's probably where your biggest buyer base would be.
Also, pictures will help a lot.
Hope this helps