I like the hostel here: http://www.hostels.com/maps/Le-Gite-du-Plateau-MontRoyal/8840 Around $30/night (for a dorm), and you're super close to all the nice things about Mtl--the new library, the Plateau, St L/St D bars, parks, etc. And within 3 minute walk to subways.
Both times I've been to Montreal I stayed at the Hotel Lord Berri - http://www.lordberri.com/ - It's only one block from Berri-UQAM, which is Metro-rific.
I always stay at the Hotel Terrasse Royale in the Côte-des-Neiges area. It is close to the Cote-des-Neiges metro (subway) station. See http://www.terrasse-royale.com/welcome.php?lang=en for more information. The rooms are inexpensive and have kitchens.