Hi, I had a quick look, and it appears that autoeurope.com has some nice prepayed deals. They may say there is a huge drop off fee, but it is worth a shot. For what it is worth, I got this:

Chevy Cobalt 4 door with ac or similar Compact, Automatic

Rate: 260.00 USD Inclusive

Gold Rate - Includes 3 additional drivers and tank of fuel

Pick-Up Details:
Toronto, Canada
6 Oct 2008 @ 10:00
Toronto Airport Drop-Off Details:
Montreal, Canada
10 Oct 2008 @ 10:00
Montreal Int'l Airport

Please Note: Certain restrictions may apply when picking up and dropping off at different locations, including additional fees. If fees apply, we will contact you to advise you of these fees, and will not confirm your rental until you have provided payment information and accepted these fees. If fees do not apply and you have provided payment information, we will confirm your booking and send you a confirmation voucher.