Scorereader is correct that most of the sit-down-dinner-type restaurants near the Reagan Building are of the upscale variety, but there are some relatively inexpensive alternatives: places that are either inexpensive in general or at which you can dine at reasonable cost if you’re careful what you order.
Jaleo is about a 10 -15 minute walk from the Reagan Building and serves delicious Spanish Tapas, which are appetizer-sized portions of various dishes, for prices generally ranging from 4 to 8 dollars each. My wife and I have eaten there often, and we’ve never been disappointed. http://www.jaleo.com
If you’re willing to take a 20-25 minute walk to the area around the Verizon Center, or hop a subway at the nearby Federal Triangle station and ride two stops to the Gallery Place-Chinatown station on the Red Line, a number of other worthwhile possibilities present themselves.
One of them is Matchbox, where you can dine inexpensively on very good pizzas, salads and sandwiches, or spend $19-28 dollars per person for full meals, depending on the state of your wallet and your appetite. The pizzas and the "mini-burgers," which are available in orders of 3, 6 or 9 and served with delicious fried onion "straws" are especially noteworthy. http://www.matchboxdc.com/index.shtml
There are also several pretty good Chinese restaurants in the area, including Lei Garden, Tony Cheng’s Seafood Restauant, Eat First and Chinatown Express.