In Feb 2011, rented mini van for 24 days at Pearson thru Alamo for $650 (no insurance) total with unlimited km (Insider's club which is free gave us additional driver free). It even stated on standard contract that vehicle could be driven in US. As you probably know, check the rates frequently as when we initially booked it was $1500 and went down about 3 weeks before trip. We just picked it up at the airport so didn't have to pay to park car.
For feb 2012 for 24 days prices for mini vans are about $1600 but hoping they will come down like last year.
Dollar & Thrifty also allow to take into US and were cheaper but limit km to 4800 and then 200 km/day ($0.25 after) so when calculated extra km about the same price. (we drove 5600 km for trip to Orlando, Cocoa Beach & Clearwater).
I did a far bit of research on the internet and used the talk online feature to ask bout driving into the US from Canada to get additional info. I priced renting off the airport to avoid the airport taxes but found either more expensive or didn't allow driving into the US.
I also checked Buffalo but for mini van was about same price and then would have to pay for parking after that.