Used vise prices and availability seem to vary a lot with location. In some places there are quite a few available and others nearly nonexistent.
Around here $100 will buy you a solid US made vise pretty easily if you know what to look for. Wilton, Columbian, Reed, Erie, Yost, American Scale, Rock Island, Hollands, Athol, Athol/Starrett, Prentiss, Chas. Parker, Morgan, Ridge/Rigid, and various Craftsman vises are all names to look for. Just know that even the "good" brands have had junk vises made overseas, so it's best to do a little research.
This site has most of the brands with pictures for most of the models....really handy reference: http://scuttle.dayid.org/wiki/index.php/Main_Page