The STM 747 shuttle to and from the Trudeau Airport (YUL) was awesome in that it was easy to figure out and inexpensive. For a 3-day pass or ticket, it will set you back $18 CAN or $13 US. So at a minimum, $6.50 is your one way cost into the city and back out again.
The buses are huge with nice storage at the bottom and comfy high seats above with tinted windows and good AC. Drivers are nice and will help with your luggage.