IAD Transit
Since this discussion has gotten onto the subject of IAD, when I was there I found that Metro express buses go the airport from Rosslyn with a few stops and free transfers. The fare is about $3. The Washington Flyer takes you to a Metro station and charges about $10. There are no signs in the airport indicating where the Metro bus stops but if you ask someone who works there, you can find out - there's a small bus terminal near the far end of the main terminal curbside. And the best meal is at Five Guys. But I guess for the best price, BWI wins. And it's a lot nicer as an airport.