It depends on a lot of things, but being defaced with someones name it is purely a shooter. I wouldn't pay over $250-300 for it and only if it has original Model 12 stocks and it is a round butt. 4" Model 12s are far less common than 2", and most 4" guns I have seen have been square butt. My preference is round.
I have owned 5 M-12s, 4-2" RB, and 1-4" RB. Still have 4 of them. The 4" was the only one I had seen when I had the money at the same time! I really wanted a 4", RB, Nickle M-12, but I have only seen one in the past fifty-two years at a gun show, and I didn't have the money.
Someone is sure to warn you about cracked frames on M-12s. This happens, but far less frequently than many will claim! Even steel frames have been known to crack! Personally I have never seen a cracked M-12, and I have examined hundreds of them since the early 1970s.