With regards to parking downtown, I would recommend 2020 University indoor parking garage. On the weekend it is only $5 per day. It is directly connected to the McGill Metro station.
List of rates (this company also has other parking garages/lots):
https://www.vincipark.ca/recherche/d...university/228
Parking on the street is definitely more expensive and most places downtown or in parts of the Plateau you have to put money in the meter until 9pm on Friday, and 6pm on Saturday/Sunday. (the city's street parking website:
http://www.statdemtl.qc.ca/index.php?page_id=5&lang=en You can also now pay for parking on your iPhone) Generally speaking it is about $3 per hour. I believe there are free on-street parking spots in Griffintown, south of downtown Montreal, although they get taken quickly.
I'm not familiar with most outer stations aside from Longueuil. The case varies a bit everywhere. At Longueuil the lots are owned by the city, whereas in most other places they are owned by the AMT (which is usually free for transit users but I think requires a permit) or private lots.
Longueuil (2,300 spots): You can leave your car in outdoor long term parking in P1 ($8.50) /P2 ($7.25) per day.
http://www.longueuil.ca/vw/asp/gabar...9571&cat_rac=7