Unless you can get a better fare on AC direct to Canada, you are facing the expense of paying US INS/customs/ag fees twice and those and other fees and taxes on a ticket from ORD -- likely your best UA gateway city from which to reach YYZ -- will be over $100. So even a cheap fare will be 30% or 40% more. [UA has had YYZ-LAX-YYZ fares at around C$275 all in, so this fare may also be available ex-LAX.] Driving from Buffalo is not a preferred option, particularly if you have just travelled a day-and-a-half, done a domestic US transcon and connecting flight to get to BUF, and then must deal with driving on the wrong side of the road. And by the time you have paid for the rental, you will have covered the cost of flying from ORD to YYZ.