Check out the Best Western Rosslyn / Iwo Jima. It's close enough to the Rosslyn metro, and it's in a safe place. Plus, they have microwaves and refrigerators in their rooms. Their rates are usually $99-130 per day. Check out Expedia for one of their deals....also check out Inns of Virginia and Days Inn Pentagon for hotels as well, they're in the same price range.
Look at craigslist under Sublet/Temporary, too. You might be able to find something cheap there, too as alot of ppl. will rent out their place pretty cheap for a week or two.
Also look at Bed and Breakfasts within Washington, DC. There are lots of them and their rates aren't too bad, but with the election coming up, things might be getting pretty booked.
Don't look at SE and NE DC (no offense to anyone here) - try to stay in Arlington / Alexandria or NW DC if possible.