Welcome to canada.
I did a few quick dummy bookings and found the following:

20 Jul for 7 days CLT to YYZ $500 USD on Expedia for most airlines
same dates CLT to BUF $300 on expedia
Same dates CLT to YYZ on aircanada.com $720 (all fees ect included)
Same dates on CLT to YYZ on united.com $551 (all fees included)
Same dates CLT to BUF on united.com $279 (all fees included)

It does seem cheaper to fly to BUF and drive into Toronto. However you should consider that it is a 2 hour drive from Buffalo to Toronto before you factor in the traffic on Canada's busiest highway, and the potential for long lines at the border. I would plan for a 5 hour drive.

If you have the time, your drive from Buffalo will take you past Niagara falls and through ontario wine and peach country. If you are a tighter schedule or on some kind of business trip, I would recommend flying into YYZ.