Ballston area.. cheap eats?
el pollo rico is the default bargain restaurant for that neighborhood. a meal is under 5 bucks for the best peruvian chicken around.
ravi kebab has the best kebabs around and they start at around 8 bucks, with enough food that they can share a meal.
vapiano is pretty good and not too expensive.
seconding the vietnamese recommendations below as well.
also el charrito over in clarendon has delicious tacos and pupusas.
El Pollo Rico in Virginia Square for Peruvian chicken.
Ray's Hell Burger in Rosslyn.
Many choices for Vietnamese in Eden Center.
Cafe Tirolo for Austrian food.
I've heard that there's a wonderful Italian food cart in Ballston during the day, but I haven't had the opportunity to try it.
The pupatella food cart in Ballston is great. Eleventh Street Lounge and Eat Bar have some great cheap eats, especially when Eleventh has happy hour specials.
Italian Store for subs. Lost Dog Cafe and Earl's Sandwiches for sandwiches and pizza and stuff. Look up a thread called something like just moved to Cherrydale and print that for her, Cherrydale is a nearby Arlington neighborhood and that thread is huge.
Also Grand Cru has great lunch, their small plates aren't all that small and are very good, as are their sandwiches and desserts.
Tarbouch, Lebanese Butcher and Lebanese Taverna Market are all good Middle Eastern.
So we're stretching "Ballston" to include Rosslyn, Courthouse, Clarendon, Lee Highway and Eden Center? If so, that's fine, but I don't want the OP to have the idea that all of these places are within easy walking distance to Ballston.
OP - you have gotten some great suggestions. Just beware that many of these places aren't in Ballston.
On the limited income note- I would add half-price burger Monday at Whitlow's on Wilson (one Metro stop away and often a wait for a table), half-price specials on prepared food at the Harris Teeter, e.g., tuna sub on Friday, rack ribs on Sat. and add Dona Azucena's on Glebe Rd just above Rt 50 for very inexpensive Salvadoran fare, but be aware that sometimes no one speaks English and the juke box can be really loud.