I live in Montreal... and it is affordable, if you compare it to cities such as Toronto.
It all depends on which part of the city you want to live in. If you don't know French, you'd be better off on the west side of the city... You could probably find decent affordable places in neighborhoods like Côte-des-Neiges and St-Henri... Check out Notre-dame-de Grâce too (commonly known as NDG)... Prices are usually higher but you can still find some good deals... Of course, I don't know what your standards of living are. Do you plan on living with other people?
If you're an immigrant, I'd tend to tell you to really check out Côte-des-Neiges... It's very multi-ethnic.
I live with 3 other people and it costs me 325$ rent per month... add transporation, phone and electricity bills and you can make that 425$... And I live in a very decent place. It's far away from downtown, but I'm so close to a metro station that it takes me barely 20 to 30 minutes to go downtown depending on the moment of the day...
If you're not afraid of French, and a little weird action at night time, you could find pretty cheap places in the Eastern part of the city... Hochelaga-Maisonneuve is a typical example...