The answer to the question will completely depend on how quickly the person wants to get from Buffalo Airport to the Toronto Airport, and how much they are willing to spend.
The options below range from a Taxi which is the most expensive, but quickest, to bus and rail that will be cheaper, but take longer.
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"Niagra Falls Taxi" has a list of pricing, including: Buffalo to Toronto Airport area $168 USD
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I live in Toronto and would NOT recommend taking a US based taxi from Buffalo to Toronto airport. Way too expensive.
The MUCH better way is to take an airport cab to the Buffalo Greyhound bus station, then go by Greyhound bus to the Toronto downtown bus terminal and then get on the Toronto air port express bus that goes every 20 minutes to the airport. Travel time....Buffalo to Toronto 3hours, and 30 minutes to the air port from downtown terminal. Cost in total about $70 CDN. Which equals about $50 USD.
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Amtrak: $80 US There is one train per day from the Buffalo airport to Toronto. Leaves at 3pm from Buffalo and arrives in TO 8pm. From Toronto to Buffalo, it leaves 830a and arrives 110p. The Depew bus station is the one closest to the airport.
Greyhound: $70 US There are multiple buses throughout the day from Buffalo to Toronto.
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Buffalo Airport Limousine Service, $160 to Toronto airport or downtown.
Their price can seat up to 4 people with luggage.
Gtaa Taxi Toronto also offers Toronto to Buffalo Niagara Airport at $160.00.
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Check out Megabus.
They go around 2-3 times a day from the Buffalo Airport to Downtown Toronto. Check out the website as I've found the bus prices to fluctuate anywhere from 9 - 30 dollars. From there, you can use the TTC options to the airport.
Megabus is by far the cheapest way (compared to 175 for taxis etc) you can also get directly from TO downtown to Buff downtown or BUFF Airport -so that's great - their site megabus updates info- BUT BEWARE with border crossings it can take a long time on any type of transportation -especially when they have to clear 20+ people before you can continue- so plan accordingly.
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Greyhound busses now go directly to the Buffalo Airport and take you to Toronto (with stops at Buffalo greyhound station, Fort Erie, Niagara falls, St Catherines and Grimsby).
The Greyhound schedule is more often than Megabus and competitively priced (if not on a 1$ Megabus fare).
See www.nfta.com for Buffalo Bus schedule to airport in addition to buss from Toronto to Buffalo Bus station
Technically, Buffalo airport would be the nearest but there are no flights from Toronto to Buffalo.
We need to travel from Toronto to Buffalo Airport on 5 Sep and Buffalor Airport to Toronto on 6 Sep 2009
Walk
Take the megabus! It leaves from Downtown Toronto and goes directly to Buffalo Airport.
take amtrack http://www.amtrak.com/servlet/ContentServer?pagename=Amtrak/HomePage
106 miles
http://gocanada.about.com/od/canadatravelplanner/a/buffalo_airport.htm
Walk.
The quickest and easiest way to get to Buffalo from Toronto is the QEW Hamilton, which changes to QEW Niagara. Take that to the Lewiston Queenston bridge and enter the United States, following the I-190 South until you reach I-290 East. Take the I-290 to the I-90 West to Exit #51E, Route 33. Take Route 33 right to the Buffalo Airport. The Buffalo Airport is about 2-2.5 hours from Downtown Toronto.
Toronto's Bus Depot ('The Toronto Coach Terminal') on Bay Street is within walking distance of the Delta Chelsea. There is, I believe, bus service direct to the Buffalo Airport. Call the Bus Depot: 416-393-7911.
Either public transportation or a taxi.