According to J.T. Callahan, Savage, Stevens, Fox historian, the 5100 is basically a Model 311 or 5151 with a slightly different frame design that was made between 1941 and 1948. A lot of the early Stevens sxs's with 28" barrels were choked mod/full. They were also sold by Sears marked as Eastern Arms 101.x
If there is no side to side or up and down movement of the closed barrels with the forearm off, tight at the face, and also no rust or pitting in the bores, I think it should be worth at least $250. I might even consider $300 if it weren't for the camo "paint."
I have a 16 ga. 5100 in really nice shape with a wood stock that I got for $300. I shot my first 25 straight EVER with it shooting wobble clays where most of the shots are 20-35 yards. It is heavier that most of my other O/U's and sxs's, and I am partial to 16 ga. guns.