as BamaVol said, the onsite garage is reasonably priced (I think $11 or $12/day). But for trips longer than a day or two, I park at Park Place right on Snow Road (next exit after the Airport on 237). The AAA rate (and internet special below) is $7/day - includes a bottle of water and USA today upon arrival, and free car wash upon departure. It's very convienient, the shuttle follows you as soon as you pull in and waits for you to park, then takes you directly to the terminal. Sometimes this can be quicker than finding a spot in the garage and walking over the bridge to the terminal!

Check out http://www.parkplacefree.com/parkplace/home.aspx