I think with the dollar being so good, lots of people are doing back to school shopping in the US, and filling the BUF hotels. We just got back from a last minute overnight trip to Buffalo, and we had a hard time getting a hotel at the last minute. All of the Marriott family of hotels were fully booked up, even for Marriott rewards members, so there's no way that one of their hotels would have been available through Priceline.
Have you tried the Sleep Inn or the Comfort Suites? They're usually pretty reasonable and they should have rooms that accommodate 5.