WestJet is having a sale which must be booked by July 10 for travel through Oct. 31. Click on their site, choose your language, and then on "Sizzling summer sale" for further details. While hardly cheap, these fares are lower than normal.
Montreal to Vancouver is set at $518 CDN plus tax for example. Interestingly, when we searched for the lowest fare on this route via travelocity.ca, we got a fare of $571 CDN before taxes, so it appears WestJet is selling seats on its own site for less than on third-party sites, which, of course, is a growing trend in the industry.