The economy lots are $9/day, which would soften the blow a little. They significantly increased the size of the economy lots, so if you're not flying out on the Wednesday before Thanksgiving or perhaps the Tuesday before, you should be OK. I live in Leesburg and I've PM'd you with some more detailed questions I didn't want to put on the open forum.

Link to Dulles parking: http://www.metwashairports.com/Dulle....htm#Long-Term