You can always go to one of the Peruvian restaurants in town. Solymar on St Hubert is one of them. We always order the jalea (mixed fried seafood), seafood soup, ceviche... It's best to go with friends because their portions are large and made for sharing. I find it quite reasonable. We never pay more than 25$ per person after tax and tip.
7610 Rue Saint-Hubert, Montreal, QC H2R2N6, CA
Try Poisonnerie Rayan. I've only been to the one on Cote-des-Neiges but there's another location on Belanger. Fresh fish & seafood, large selection, cooked a few ways depending what you order - mainly grilled/oven baked/deep fried. Maybe more fish than shellfish, but it depends what you're looking for - they have calimari, shrimps, and some other shellfish I probably forget. Extremely reasonable - $6/lb. for calimari, for example. You order at the counter in the back and pick out your raw seafood, and they bring it to you cooked the way you asked for it. It can be crazy busy so try to go in off-hours.
Edit: sorry, the calimari was $7/lb. last time I was there, not $6
I also strongly recommend poissonerie Rayan and it is more than reasonably priced. Barbequed, deep fried or grilled whole fish, deep fried calmari, garlic and olive oil sauteed shrimps and octopus and deep fried skate wing with a side of fries and large salad is what I try to get every time I go there. Try the hot sauce too. I really feel like it fits what you are looking for perfectly.
What is reasonably priced ?
There's the new restaurant from Chuck Hughes called Bremner in Old Montreal; no real menu can be found on the web (same for his other restaurant).
Le Filet on Mont-Royal (from the Club Chasse et Pêche gang), probably more fish than seafood ?.
Au Pied de Cochon for the seafood platter.