I live in Capitol Hill and there are so many options it all depends on what you like and who "we" are. Couple? or are you bringing kids?
I love the food trucks, but $15-20/meal isn't my idea of inexpensive. Not sure you can find a complete "meal" for less when eating out, but usually kids don't want a "meal", if they'd rather just have a sandwich.
Do you love food and the experience or do you just want to eat for energy? Do you want to combine sightseeing with buying food? Eastern Market is the longest continually running farmer's market in the US, and you may want to buy some of your staples there.
Do you want to combine an "eat-out-meal" with one of the museums? In that case, I would plan an eat-out-lunch with a visit to the Museum of the American Indian -- consistently gets the best reviews of all the museums and the food is based on food available to different Indian tribes.
If you like the idea of government cafeterias, they can be great, but I would guess that Dept of Ag was cited because a bus driver can park/idle his bus in that area of the mall. I've been to that Caf and eh...there's something for everyone, but it's a Caf. As an alternative, I'd suggest the cafeteria in the Longworth Office Building. Much less expensive than eating at any of the museum cafs, and an incredible selection. I do NOT, repeat NOT, recommend eating in the cafeteria for Voice of America.
I also love the cafeteria at the Newseum and if you're a family or a couple, buying a membership ahead of time can pay for itself. I think you might also be able to get a library pass for the Newseum.
If you're a foody, I recommend the chef's counter at ChiKo (coordinate with your trip to Eastern Market). For $50 each, you get enough food for 3-4 meals while being able to try a little bit of just about everything on their menu. I love chef's counters but feel they are usually too expensive for me. This one was affordable for a special night.
I love Ethiopian food too! Lots of great choices at great prices. (my favorite is Ethiopic on H. Street). The Wharf is new and fancy, but expensive. You may want to walk through the Wharf, see the sights, experience the tourist-places, then go right next door to the original Maine Avenue Fish Market (also renovated, but still much much cheaper than anything at the Wharf) and buy some fresh mussels or crab and have them steamed right there for you.
Tell us more about what you want, and many of us can provide more recs.
One other suggestion: include the option to use bikes and scooters to get around. There are bikeshares and dockless options ALL OVER. You can get so much done and have fun when you're on a bike or a scooter. You may want to download apps for CapitalBikeshare, Jump, Bird scooter, LimeBike. So much fun!!! If this is an option for you, bring your helmets with you.