Sushi Inbox is the best 1 $ sign sushi in downtown Montreal. It's always empty, but the sushi is pretty good for the price, and I'm not picky.
The only problem I have with this place is that going to the restroom is weirdly complex - you take the key, walk down a really long corridor and try to figure out which of the three or four doors it unlocks. Also, the light didn't work last time I was there, there were exposed wires and heating pipes, and it felt like peeing in a cupboard.