There is the Econoparc P6 to P9, which costs $30 a day and is directly on the site of the Montréal-Trudeau airport. This parking lot is on 3 levels, and those lucky enough to be able to take advantage of sheltered parking. Otherwise, the top floor is open-air.
Park'N Fly, on the other hand, is open-air. Park'N Fly is off the airport site and prices start at :
$ 22 per day for self-service parking, including a 10-minute shuttle service to the airport
$ 24 per day for Valet parking, where everything is taken care of, right down to clearing snow and plugging in your electric vehicle. The shuttle takes you to the airport in less than 5 minutes.