I looked into this because I will be going to Whistler soon.
offers the cheapest round trip fare at about $70 round trip. They also offer packaged discounts when you buy lift tickets with them. I am not using Snowbus because the bus schedule doesn't coincide with my arrival flight. Plus, there will be others, so renting a car for all of us is cheapest especially when we can do our own groceries and get to see a bit of Vancouver in the process. Perhaps, do some shopping at Park Royal Mall on our return to the airport. You need to book in advance
Greyhound from downtown Vancouver is the next cheapest and it's $100 round trip. We weren't interested in taking the Skytrain to downtown Vancouver (additional $8 per person).
There is Pacific Coach from the airport but that's $140 round trip.
Taxi one way flat fare is $250 and needs to be reserved in advance so round trip will be $500. Renting a car makes more sense.
I know you said no cars but the other cheaper option is zipcar. They have services in Whistler and Vancouver and the rates seems okay. However, again, I'm travelling with a group so renting a car still ends up being cheaper.