Your best source is www.mwaa.com.

I generally taxi to IAD from Reston. $25 one-way. At $9/day for the economy parking two taki fares are amortized in a one-week trip. From DC the fare is about $50, so you break even on a two-week trip.

If timing is tight a taxi is right. It takes about 30 minutes to get to the terminal from economy parking. 45 min if you're unlucky.

I use Fairfax Yellow Cab 703.534.1111. You can set your watch by these guys.