Which is the least of it. There is anywhere from 1.5 to two hours on either side. And the additional $200 each way, given the distance of the airports from the city center.
However, even if you want to get to the airport early, it's not like that time is wasted. These days I would say relaxing in the airport is just as nice as relaxing in a train car: http://www.nytimes.com/2014/12/08/nyregion/improving-ground-.... The last time I was at LGA early I had a huge table to myself and the person I was with, and the table had an iPad I could use to order food and drinks.
$50 each way from YUL to wherever.