What the cheapest rate for CLT to BUF RT. Flying to Buffalo will be cheaper than Toronto.

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.

I often use BUF instead of YYZ - particularly when on vacation with the family. You can typically save up to 50% as the example above shows. For one person however you need to factor in whether you are saving enough money for the hassle and the cost to get to/from BUF. For 2+ people it almost always makes sense. There is a bus from BUF to downtown Toronto which can make the transfer less painfull.

From BUF parking to my home in North end of Toronto is about 80 minutes driving at the flow of the traffic. CBSA border varies from 5-20 minute for me most of the time.
There are times that the wait is over 90 minutes during the border crossing rush hours.

5 hours driving from BUF I am almost at DTW.

If you are flying into BUF plan to make the drive through wine/fruit country enjoyable. Niagara Falls, although overly commercialized still has the falls. There is also quaint little town off Route 55 from QEW call Niagara on the Lake which is wonderful for a day trip.

The BUF to downtown bus is a low cost way of getting to Toronto downtown if you can deal with your luggage. It never worked for me for I do not live downtown and the bus schedule is not great with only 3 buses/day.

If you can get a good deal on a rental car and are willing to pay for parking, you can travel via BUF but there are added expenses and time. There are times when fares to YYZ get lower - depends on the competition. Buses add at least three hours to the trip and some considerable uncertainty about catching your flight on the return. Megabus Canada's service from New York to Toronto stops at BUFand so does the Greyhound NEON service, but both require advance reservations and are only a few times a day. There is a transit bus from BUF to the downtown terminal where Greyhound and Coach Canada have service but that adds a lot more time, and cost. Flying to BUF might be cheaper but that's far from the whole story. What they lose on the swings, they make on the roundabouts.

Cheapest would by CLT-BUF + Megabus, but it will depend on how much time you have. If its a quick w/e trip, I would rather do the flight to YYZ, if its 5 days, you have time, rent a car, see Niagara Falls on the way up.