Parking on campus is permitted on the weekends. Enter at the Roddick Gates on Sherbrooke St (opposite McGill College Ave). Take a parking ticket from the automated booth at the entrance (there may also be an attendant, esp on Saturday). You must pay when leaving, via the same gate, either via the attendant, if present, or via the automated machine, which will accept cash or credit cards (if cash, the machine won't accept the new $20 bills - yet! - and of course won't accept US cash). The cost is $7 per day, flat rate.
Editorial comment: this is a very reasonable rate, and is far better than any local commercial parking lot. It's also a lot cheaper than parking on the street and getting a ticket! If you come early, you shouldn't have trouble finding a spot either, as Montrealers usually are a bit slow getting going on the weekends! (Just don't plan to drive away at lunch time, as you'll have some trouble getting your place again afterwards!)