Empress, I do YYZ-MIA-YYZ all the time and I haven't found any cheap way of doing this trip. I'm typically paying about $1500. The fare for this route is ridiculously high. With the exception of the occasional websaver I always buy unrestricted fares. Even the 7 day advance booking discounted fares are quite high. I'd be interested in hearing from you if you find a cheap solution. Good luck!

US customs might be interesting. They could regard this either with suspicion or amusement. (Although INS officers aren't known for their sense of humour.) You might get a grilling but I'm sure they won't deny you entry.