We always fly out of SEATAC and we follow the same routine every time. It works for us and we have peace of mind. It may not necessarily be the cheapest option but we want our car to be in a 24-hr secured lot, given there are a lot of reported break-ins in the SEATAC area.
We use Priceline.com to get a 3* and higher hotel in SEATAC area. We always get the Doubletree SEATAC Airport from $45-$60, depending on time of the year. It's a big hotel with big rooms and just perfect for the overnighter. Their parking is $18/night. But we factor that into the cost of the room.
On the morning of our departure, we go to Master Park Valet Garage (just down the street) and they take care of our car. We take their shuttle for the 5-min ride into the airport. Master Park Garage charges about $12.99/day (use the online coupon they have on their website).
For our upcoming 9-day trip to Hawaii in February, we anticipate the total cost of our SEATAC stay and park to be ~$170 USD. Still MUCH cheaper than flying out of YVR!