It depends on quite a few factors and can be very hard to predict.
Weather
How the Bills are doing in the first 2 games
How the Patriots are doing in the first 2 games
How many tickets are left / how many people can't go
I often buy gameday Yankees tickets for well below face value. The key is to keep an eye on the # of tickets remaining (and sometimes the weather report), as this number goes down, prices will usually go up. If this number stays relatively high as gameday approaches, especially if lousy weather is predicted, people who can't get to the game will usually start to become desperate and lower their prices. Keep an eye on the Oakland game the week before (although it's not a division game, you may see some patterns which could be helpful.) Of course, your mileage may vary.