Sorry, there is no metro to YUL, you can take a taxi, it cost 24.75 to go into the city of montreal, there is the airport bus that will take you into the city and right to bare quam, it is a main metro, i forget the cost of the airport bus, or you can take BUS number 204 right to Dorval bus terminal, and the take the 211 to the end and it is a metro stop, it takes about 15-20 minites from Dorval, you can also take the 202(dawson metro) 191(LONG LONG LING bus ride,, same stop as 211) but your cheapest way is :
204 untill dorval bus terminal(5 minites) then,
211 untill downtown
all this will cost is 2.00$ . but if you have 30$ to spean take a taxi, but if it is traffic time,, take the bus....
Oh you can also take the train from Dorval terminal,, but i dont know the time it passes from..
hope this helps