One extremely low-cost option is the Herndon-Monroe Park & Ride, a public lot located a few miles east of IAD off the Toll Road.

Parking here is absolutely free and there's no prohibition on overnight parking, unlike in Metro station lots.*

From the park & ride, hop on Metrobus 5A. The bus takes only 8 minutes from the park and ride to the curb in front of the terminal---fare is $3 per person.

The biggest inconvenience to this route is the bus service. The 5A only runs once an hour, from roughly 6 a.m. to 11 p.m. and tends to be very crowded with very little room for luggage. But hey, it's cheap. And the buses from the $9 a day long-term parking at the airport are PAINFULLY slow, stopping at every shelter in the designated lot and going a steady, law-abiding 25 mph on the roads to the terminal. As long as you arrive at Herndon-Monroe reasonably close to the bus's scheduled departure time it'll probably take just as long to get to the terminal as it would on the long-term parking buses.

*Disclaimer: I've never actually attempted overnight parking here, but I've passed through the lot on the 5A bus and there are no signs stating a time limit, nor is there any mention of one on the Web site. I am, however, flying out of IAD on Tuesday (only for one night) and I'll park at Herndon-Monroe and share my experiences in this thread!