The problem I have with leaving ex-SEA instead of ex-YVR is that it basically eats up either a work day or a vacation day. I can get a 7 PM or later YVR flight with no difficulty after working all day. I can't do that with SEA. So not only do I have to think of the savings ex-SEA, but of the value of the lost time.

The calculations will be different for people with a different schedule, obviously.