I wouldn't pay more than $250 or so. $300 at the most. I have noticed that "diesel guys" price their used parts quite a bit higher than "4x4 guys", so i do see d80s for those prices now and then, but would never pay that.
The fronts are awefull. The use unit bearings and are low pinion. The Dana 80 is a monster for offroad and a 14 bolt or 70 is plenty for most of us.
Dana 80 - $100 to $250
Dana 60 - $200 to $400