In my travels i have run across an old HG-58 or 68? sitt'n in an old barn. the old gal hasn't been run in who knows how long, but the sheet metal is in good shape and the bird droppings have insulated it from the elements. What is a fair price to give for a cletrac project tractor? and we'll call her in rough to salvage shape.
You've probably already read this http://www.cletrac.org/newbb/viewtopic.php?t=975 (http://www.cletrac.org/newbb/viewtopic.php?t=975)
I would say an HG would be worth anything from a couple hundred bucks (if you can't get it for free) if it doesn't run and the tracks are shot, up to a couple thousand if it's in good shape. Maybe more if it's a super tractor. All depends on condition, where it is, and who you're buying from. How bad they want to sell it and how bad you want to buy it.
No set rules here.