American Airlines and Delta Airlines , combined is offereing a rountrip airfare of $172, including taxes from New York City (LGA) to Montreal, Canada (YUL) for travel October - August 2017.
Delta's one way fares from LGA to YUL is $67 on most days.
American's one way fares from YUL to LGA is $105
Check date availability on Google Flights and use those dates & flight information to book separately.
My sample dates are: June 18-25, 2017 (1 week vacation)
Please note that while this fare is valid at time of posting, fares can fluctuate in a matter of minutes/hours/days.