Nilufar is a great place for when you're near Concordia, and you don't want to spend a lot of money. I don't think you can get a cheaper falafel pita in Montreal. The food is actually quite good. And the woman who usually works there is very friendly.
The food is good, and a little surprising. For example, they have a few middle-eastern soups, and they seem to be homemade. That's extremely rare for a place in this price range.
This is one of my favorite places to eat lunch.