French near DuPont Circle
Bistro Du Coin is in Dupont Circle and it does classic French bistro food very well. There is a range of prices on the menu but you can definitly get a meal for one person for around $15-$20...probably even cheaper if you just get a salad and their wonderful French onion soup.
The atmosphere is fun, although it can get a little noisy when everything is in full swing.
I eat there a couple times a month though and love it.