What would be the value of a Leupold M8-3x. Very good condition.

I understand that this model (which hasn't been cataloged for quite some time) is (or recently was) available through the Leupold custom shop. It is highly prized by shooters of large caliber rifles and other rifles made for close encounters. I don't think you'd have much trouble getting north of $200 for a nice used one if marketed broadly enough to find the right buyers, and maybe well north of that number.