in Chinatown, excellent basic viet/thai can be found at Crystal no. 1 on St-Laurent between La Gauchetière and René-Lévesque. best soups NW corner Viger/St-Laurent. best viet submarines on St-Laurent too, west side, between René-Lévesque and La Gauchetière (don't remember the name).
for bistrot-style French food, try Le Grand comptoir on Phillips Sq for lunch - cheap tables d'hôtes.
for the best deals, you better walk east or go up the hill.
seafoods... I don't know. best fish I had was on Duluth, Le Piton de la fournaise 835 rue Duluth (close to St-Hubert). other excellent fish place : Le Poisson rouge 1201 Rachel East in front of Parc Lafontaine. La Mer, close to métro Papineau, René-Lévesque/Papineau. these three places are a bit over your budget.
Portugese restaurants serve good fish! On Marie-Anne and Rachel between St-Denis and the mountain, you may find some of them. the most famous is Doval at 150 Marie-Anne Est. good portugese spots on Ontario and Panet/Plessis too.
personnal cheap recommendations : Frites Alors (local chain with best fries and burgers) on various locations
http://www.fritealors.com/site_flash/emplacements.html
and Café Santropol St-Urbain/Duluth, super terrace with mega healthy sandwiches of all kinds.
best strips for food : St-Viateur between St-Laurent and Hutchison, Rachel between the mountain and parc Lafontaine, St-Denis between Laurier and Maisonneuve (and the little stretch of Ontario just West of St-Denis), Little Italy around Jean-Talon Market (métro Jean-Talon), Laurier between Brébeuf and Papineau. there may be many more.