I have done this a few times with the Quick Shuttle service out of Vancouver. It is about US$50 RT and has never been too painful in the past (touch wood). I usually leave on the 7.30am bus and try and book a flight out of SEA around 2pm. Including time at the border, I am almost always at SEA by noon. The hoss is coming back, with the last bus at 7.30pm. But given what is often an outrageous price differential between YVR and SEA it is frequently worth the trouble.