We stayed at one of the overflow hotels and we bought a MARTA 4-day pass for like $6. I wish DC's subway system was that cheap. Now the only con is that if you want to do a late night event, you'd have to take a Cab back to the hotel since they close at a certain hour.
Hotels are booked, but in previous years the hotels will offer blocks in the spring or summer. Hilton did last year.
I also check this blog out, because sometimes you can get good deals and she searches for them. http://xiandesi.livejournal.com/
You can use the dragoncon lj page also to look for room shares as well.