The 121 M-1 is one of the best shotguns ever made, IMO. I have two of them. In good condition, I would expect the value to be $500 or so. One of the joys of this particular shotgun is the heavy wood stock soaks up recoil. Stick an aftermarket slip on recoil pad to absorb more, and it is an easy shooting gun. It likes heavy loads though, High Brass birdshot and .00 buck, full loads.
Keep it lubed up, and learn how to take the bolt apart and clean it. Also, it operates on a delayed inertia system, meaning do not take a hold against your shoulder, as that prevent the recoil from ejecting the shells.