I think Bret summed it up pretty well.
The bottom end price is fixed by scrap value.
Yeah - 500's on the higher end of things.
But, event at that price - the amount you could negotiate downwards before you hit the scrap price isn't really enough to make or break the deal if it's something you really want.
as with any old machinery - as long as it's in working order (give or take) and has all its critical parts (especially the tough to find, tough to fake parts).
and as long as you're not trying to make a profit on restoring it - I'd buy it.