4" are very common. I paid $27 for one at a flea market. But my 5.5" I payed $100 for, and that was a pretty good price for as nice a vise as it is. I've never seen a vise that big... The big ones are exceedingly rare, most of them got melted down for scrap. $750 probably isn't all that unreasonable, really. It's not a GOOD price, but I wouldn't be surprised if he could get it, as long as someone was local who wanted it...