And, if you're forced to hire someone to drive you, taking a TOWNCAR SERVICE is actually almost always cheaper than a taxi if you're going farther than downtown...something I learned the hard way. The Town Car/Limo services provide a flat fee based upon zip code of your destination, whereas the taxi is the standard mileage plus time plus airport fee. So I took a taxi 6 months ago up to my family's place in Bothell (as it was 3am) and it was over $85!! (The towncar rate for this trip is just $60).
There are also shuttle vans...i think the shuttle service counter is open until 2am. It's around $30 to shuttle to downtown, I believe. Although I've heard that the taxis to downtown have a FLAT RATE so maybe that's the way to go? *shrug* I'd check out yelp or something similar and call a few taxi and towncar (and maybe shuttle) companies to check their rates. Be sure to let them know what TIME you'll be going too...some of these companies have a surcharge for late night/early morning hours.
Good luck