Public markets and small independent grocers really save the day if you're looking for organic stuff without breaking the bank. In Montreal, Jean-Talon Market and Atwater Market have a good number of local producers selling directly, so there's no middleman markup. Prices are generally cheaper than at specialized chains, especially if you buy in season. There are also small neighborhood organic groceries (like in the Plateau or Rosemont) that don't mark things up as much compared to Avril or Tau. You've also got those local fruit and vegetable boxes they deliver - like farmer's baskets - that work out way cheaper for certified organic, even with delivery.
Another thing is Costco actually, but you can't just look at the premium products. There's an organic section that's kind of tucked away with fruit, vegetables and dry goods at really decent prices, you just gotta dig around a bit. It depends on the location but generally it's cheaper than specialized organic chains. And honestly, it doesn't all have to be certified organic to eat healthy - there are tons of small producers selling organic without the official certification at Jean-Talon Market, they charge less and it's often just as good.