Off-airport parking is the least expensive, with a number of choices for long term parking with shuttles to the airport. When arriving back from Vancouver, BC, the bus or train stops in downtown Seattle. There are buses, train, or the Link light rail to get to the airport and a shuttle to your car. Here are links to a few of the parking lots—and and there are many more.
AJax Parking R Us
Way (This is a service that reserves parking at nearby hotels like the Airport Hilton, Embassy Suites, etc.)
Additionally, some hotels near the airport provide long term parking at a nominal rate, and most provide airport shuttle service. Check with the hotel or access the Way link, above.