The fares increase as the bus sells out, but I've gotten the $1 tickets a few times. Megabus accepts reservations about two months out, and you'll pay less if you buy in advance. You can also cancel and rebook up until the day of travel; you'll pay any applicable fare difference and a $1 penalty (well, not really, but you'll pay two $0.50 booking fees). So, sometimes it might be okay to book something on speculation, then come back and move the trip around as needed.

So far, I haven't found the Megabus schedule to be very attractive for those going BUF-Toronto, but it might work for Torontonians traveling out of BUF. The NFTA express bus to the airport is rather inconveniently infrequent, as well -- once tried to construct a BUF-downtown Buffalo-downtown Niagara, NY-Niagara, Ont.-TO route (partly for the adventure, partly to take advantage of quick on-foot border crossing, partly to take advantage of the plentiful TO-Niagara services), but it added nearly an entire day to the itinerary.