Cheap and yummy in DC
If you will be in DC on a Friday, you should check out the happy hour at Taberna del Alberdero. You have to order everything before 7pm but it's half off tapas and sangria at the best (and usually one of the most expensive) Spanish restaurants in town. It's only held Monday-Friday though.
For cheap local chains I like Five Guys for burgers and California Tortilla for burritos (try the blackened chicken ceasar burritto mmmmm) both in various loacations around the city.
For pizza and mini burgers, Matchbox and Ella's are both pretty affordable..
Sakana on P St in Dupont Circle has really good and cheap sushi. If you go you MUST try the Sakana Sushi Roll. Further town P St. is SoHo Cafe for a really good pulled chicken wrap or cheap coffee and breakfast.
In Adams Morgan there is The Diner and Tryst or it's sister-restaurant Open City in Woodley Park.
For Thai I would check out Mai Thai...quite affordable. And Heritage for Indian.
In Georgetown Bistro Francais has an early bird AND late bird dinner special. Also a champagne brunch deal. Not sure about exact prices, but its in the $20-$30 range per person.
On U St. Ulah Bistro is quite affordable. I liked the turkey burger.
I would not recommend Sakana for visitors. It's just a hole in the wall for locals that has decent sushi. A better bet would be Uni down the block. At least there's some ambiance and some diverse items on the menu. The Diner, Tryst and Open City have decent food but not a destination for someone who's visiting. Adams Morgan has a lot better, interesting places with comparable prices to these places. Mezze and Pasta Mia are two examples.