In my oh so wise opinion, the following are probably the best places to go tonight:
Asylum- $200 for best costume, cheap drinks anyway but free drinks, bar tabs, giveaways for whatever the staff decides
Cafe Citron- two for one drinks from 4-7 and $200 for best costume
My Brother's Place- this place is hard to find so google map it before you leave. Costumer contest winner get happy hour for 10 friends with free draft beer and $1 rails.
There are many more options, so check out the Washington Post for more ideas.