Everybody seems to be offering discounts to Cuba! This one is flying Aeromexico, with a stop in Mexico City and final destination Havana. Many dates from now until the end of 2016.
http://www.secretflying.com/2016/05/montreal-canada-havana-cuba-369-cad-roundtrip-business-685-cad/
Momondo.com is a key part of the strategy of getting this ticket price.