Unlike a number of different brands of 16 gauge, the M12 Winchester 16 gauge was built on the same frame as the 20 gauge, wood is interchangeable between the two. There are a number of very good individuals who specialize in restoring the old Winchesters but of course their quality work doesn't come cheap.
www.winrest.com is one such site...this guy does great work.
If the gun is basically a field grade M12 a lot of the value depends on the year of manf.,the condition of the blue and wood, and where you are in the US. But as someone else said, you can't buy grandpa's gun anywhere...so you have to determine what that's worth to you.
I love these guns. I currently have 3, 2 are the 16 gauge and both of these were made in 1915. So they've been around awhile, great gun to hunt upland game with...