What's a 18 1/4" completely rebuilt Rem. 1100 12 ga. with new springs, all plastic parts replaced with metal, and black stocks built by an old time gunsmith worth?
I would say it's probably a $200 - $300 gun depending on where you are in the country.
You can check gunbroker for completed listings to see if anything similar has sold recently.
While old technology, I love 1100's. I wouldn't pay the current asking price for a new one, but I would pay $350 for a nice clean low mileage example. I shoot one over the winter at skeet and sporting clays.
Great little gun but just not worth anything, you start doing custom things like sawing a barrel and stuff like that it just drops them off. Id say 250 bucks