Tailgating earns the NFL next to nothing, other than the parking fees. You're making your own food, and many people stay outside and don't watch the game. This argument will not fly with them.
There are two ways to make money - raise revenues or cut costs. The big markets excel in the former, the Bills do well in the later.
What makes the NFL money are PSLs and luxury boxes. Nosebleed seats (particularly in Buffalo) make owners much, much less. The saving grace for RWS is that, because it's old, they can look the other way on a lot of things that need repair. The heating elements in the outdoor club seats are one example. The rusting chain-link fence around the stadium perimeter is another one. A lot of other factors, too. Artificial turf is a lot less costly to maintain than actual grass, labor is cheaper in Buffalo than it is in NYC, DC, or Boston, etc. Vendor contracts are probably also cheaper than in larger cities.