There are plenty of stadiums that have as much or more capacity as Ralph Wilson, but the tickets are higher. The Redskins, Jets, Giants, Dolphins, Broncos, Chiefs, and Panthers all play in stadiums larger than Ralph Wilson Stadium, but none are the lowest priced tickets (in fact some of them have some of the highest average price tickets.
Ralph Wilson stadium's capacity is 73,967 (which is only about 6,000 more seats than Gillette 68,000 capacity, not the 15,000 you state). The county made them reduce their capacity in 1998 when they renewed their lease which means that it is probable that part of the leasing agreement is that the county buys all the unused tickets so there is no blackout. I know Oakland has (had) that agreement with the Raiders because Al Davis tried to use that point in trying to get out of his lease about a decade ago.