A cab will be about $50 and is the fastest way.If you have enough time, you can walk to the airport metro station, take an inbound Blue line train to Rosslyn, and take the 5A bus to Dulles, or at Rosslyn change to the Orange line train going the other way to the West Falls Church stop, where you catch the Flyer bus to Dulles. In the afternoon both buses run about every half hour. The 5A is likely to be faster unless the traffic on I-66 is unusually bad.
Take the super shuttle. Its not as expensive as a cab and not as much hassle as trying to get the washington flyer. You can catch the super shuttle near the baggage claim at reagan.
I hope you got there today because no one goes anywhere tomorrow.
Supershuttle. It will cost you $30