I'm sure it cost a few times that price new, but as Rory said, the market is small for such a large vise and MOST of the places who are in the market for such a vise are probably going to buy it new. The local power company has one monster vise in their shop, I guarantee they don't buy used stuff.
If you really want it I'd go there with $300 and tell him by all means the vise is worth your asking price but I cannot pay that, $300 is what I am willing to give for it. 99% of the people I have dealt with are cool with such an offer, they may not take it but they don't get pissed about it, if they don't take the offer you can always bump it up a little in a few days. If you piss the guy off he won't sell it to you for anything less than the full price. IMHO, the worse someone can do is try to beat someone up on the price by insulting their merchandise or by insulting them by acting like they are crazy for asking that price.
Wheeling and dealing is fun but too many people act like azzholes when they are trying to deal. I've had people piss me off to the point where I refused to sell them the merchandise even after they came up in price.