If you really feel you were "low-balled", it doesn't hurt to ask about it to see if it is negotiable. I work for the Dept of Navy and we have pay scales that set ranges for pay, so often pay is not negotiable. But, DC has 24.22% locality added to the basic pay. It is a nice area with a HIGH cost of living. Seems like you are headed in the right direction by filling out all your pre-employment paperwork. Have you discussed relocation assistance? Possibly they would be able to authorize a Relocation Bonus or are they placing you First Duty Orders?? Good luck!