With Seatac parking rates running at $30/day, it seems like there have to be some alternatives. When I have time (which ain't often I admit), I do use off-airport parking.
There are amazingly few sites that cover recommendations for this. Last recommendation by a human is from 2002 in google! If you can believe that. Here is what we use:
Doug Fox parking, these guys are closest and I used to use them. I don't know mainly because there are times when they are completely full.
ShuttlePark. We just used them and I have to say it is scary. When they are very full, they take your keys a valet. Let's just say many things have to go right for you to get your car back on a busy day since their whole system relies on little paper tags and keys in envelopes. They use the same shuttle as Allstar (In fact all three of these are actually run by the same company, but because Airstar is out of the way, it doesn't fill up).
Some enterprising folks from the UK even have a site called "http://www.airportparking.to/":http://www.airportparking.to/ that is a shopping engine for finding very low cost parking. Personally, I'd never use, since you want a patrolled lot if your car is at all valuable.