I grew up in BC, my family lives there, but I live in Ontario now. It's been 9 years now that I've lived here, and I can tell you that summer fares are always expensive. If you can go in June, you should be able to find round-trip for just under $500pp (Toronto to Vancouver). But July and August, they jack the rates up. I've never found flying from Buffalo to Seattle worth it for me, because we'd have to rent a car and drive to BC. However, if you were renting anyway, it might be ok. I've seen $99-139 one way on Southwest, but you have to connect somewhere, and it makes for a long travel day.
My mom wants to come visit in the summer, and she's hoping there will be a summer seat sale. They just finished one for the spring/fall, so you never know. Also, I've always found West Jet and Air Canada to have almost exactly the same prices. They do sales at the same time too. Sometimes there are deals on Sunwing, but their flight schedule is very limited. I think they only fly the YYZ-YVR route once or twice a week.
Good luck!