Greyhound bus I paid $20 for one-way 2 weeks ago. Might be a little more last-minute.

Then you can take a local bus (depending on the time) for USD $1.50 from the Bus station right to the airport (no transfers). The 24 or 204 Express will take you there. Go to http://www.nfta.com/metro/trip_planner.asp for schedule & fare info, then put "From" Metro Transportation Centre Buffalo to Buffalo Niagara Int'l Airport. There are also some shuttles that will take you for $10 that can probably be found on the Airport website. Cabs in Buffalo are expensive - it's only a 10 to 15-minute cab ride but will cost close to $30

-mike