it's a good solid vise, but probably on the high end at $100 since it doesn't have a known name. rare is collectable and could be worth a bit more if you can figure out the story and it might be worth that. Several vise companies made that design of a vise so i'm guessing one of those made them for this company or person.
my 2 cents is get him down to the $60 to $70 range or keep looking for Prentiss, Morgan, Rock Island, Reed, Columbian or any of a number of other old US brand vises if you want to make your $100 vise an investment. of course you can hope to get a Wilton for $100, but they are getting harder to find.
I'm not sure where you live, but if you were close to me i'd give you several nice choices for $100 of vises you have heard of that only need a little spiffing up like that Rex does.
good luck