That's a pretty wide open question JL. Especially without even a picture to go by.
Supply and demand, proximity to you (especially if you are in need of one), engine condition, generator condition, overall appearance, actual age (can be determined from serial #), and little things like are there any leads with it, electrodes, is it on a trailer or just the unit, etc.
Sorry, I wouldn't even hazard a guess. Hopefully some of the guys that are always buying and selling can be of more help.
He Is In Control, Have a Great Day, Brent