I picked one up in really good shape (original plastics, decals, seat cover etc..) a few months ago for $450 and I thought I did ok on the deal. However (and I should have known better) I quickly found it has a rotted out magnesium water pump housing (very hard to find used in good shape) and it needs a rod kit and piston kit so it turned into a not so good deal pretty quick lol.
Anyway unless it is in perfect/showroom condition around here I think it could fetch as high as $1300 to someone who really
wants an 86 CR125. Realistically $650-750 would move it fairly quick 850-900 might take some time. Hope this helps,