french bistro NoVA or DC (<$$)
Depending where you are coming from, I'm not sure its worth the drive, but my favorite moderate price French restaurant in DC is Bistro D'Oc. My favorite dish is their duck confit but their hanger steak, roasted chicken, and duck dishes are incredible.
Most of their entrees are rather filling and I usually just get an entree and fill up on their french bread.
Depending on what you get you can certainly get something for under $20. Or get a bowl of french onion soup and a goat cheese salad for a meal.
Also if you are interested in going to a movie, if you go to E Street Cinema first and then keep your ticket you can go to Bistro D'Oc and get 2 free dessert if you order 2 entrees. Not a bad deal.