$400-$450 maybe....but condition is only a part...barrel length, choke, Nickle steel, chamber, rib, grade all enter the picture.
And depends upon what level the buyer wishes to elevate the purchase.
Top refinishing may run $500 w/o new wood and what ever gunsmithing may be required to bring it up to operational snuff.
Doesn't need such as that, then good.
There are pecks of 16 Model 12s out there...I just bought a less than good example a month ago.
Basically, the less you want to fiddle with it, the more it will cost.
Great scattergun.