A very worn field grade gun (but still in workable condition) might get you $350 bucks. From that point on condition, guage, and grade mean everything. I just visited a web sight that had M# 12...
There's too many variables on this one. First, Second, or Third Model. Standard or Deluxe. Casehardened or Blued. I'd suggest you go to the library and find a copy of The Standard Catalog of...