The 12" versions aren't worth as much as the 6", sadly, as they really were neat pieces. The 6" is one of my favorite revolvers of all time - fantastic range and small game killing power, but in an all but recoilless package... The 6" models tend to fetch around $600, give or take a hundred bucks, the 12" version I have seen go as low as $400, but tend to cruize $450-650, depending upon the optic sold on top.
Do you have the rib mounted base, or did you add another screw to the receiver mount and D&T the receiver?