Market Value is usually determined by Three Key points:
1- RARITY. You have plenty of that.
2- CONDITION. You are hurting a bit on this point. The gun has finish loss. The left grip is obviously wrong, and I suspect the right is not original either, because it is a later round-shouldered type.
Value falls off fastest for the first few points below 100% new in the box or 100% mint. If a Mint Reg Mag in a certain barrel length is worth $10,000, it does NOT mean a 98% gun exactly like it otherwise will bring $9800. Those TWO % points of condition will affect value by more like 10-20%!!
3- DEMAND. Demand is determined by the two above points, plus the ABILITY of buyers to play in the given price range- in other words, there are many more buyers ABLE to purchase a $750 gun compared to a $20,000 gun. There is also the possibility that many of your players took a HIT in the current economic crisis. DEMAND is greatly affected by CONDITION. Many of the collectors who have the ABILITY to buy your gun won't even consider it because of condition. They would readily pay $10,000-12,000 for a pristine example NEW-in-the-Box, but they won't even consider this gun at $3,000 because it falls below their standards. Probably more than half of your players will sit this game out.
In my opinion, this gun, in this condition, at this point in time, is worth around $5,000. I am not sure it will sell readily at that, but it would be a good starting figure.
Good Luck!