When I think of eating at the airport, I usually frown... because your choices are usually limited to fast food... there are a few exceptions and here are some of my favorites.
Matsutake @ Ronald Reagan National Airport (DCA) (United Terminal)
Sashimi deluxe - $29.95 / Uni nigiri - $6.75
I have eaten there twice this year and I plan to eat my lunch there every time I am in DC. Good quality fish, fast and friendly service... what else can you ask for? (Last time I sat at the bar, the sushi chef even gave me a complimentary tuna roll!!) It's not necessarily on the cheap side but it's reasonable... and I would save my food budget for it. The only other catch is that the restaurant is located outside the TSA checkpoint so if you plan to eat there, you have to allow yourself ample time (airline status was always nice but really nice in this case).
Vino Volo @ Philadelphia International Airport (PHL) (United Terminal)
Wine flight / Mixed Olives / Smoke Salmon (the Prosciutto, which I did not ordered this time, is very good too) - I think the whole bill came out to be around $25.
I go there whenever I could justify it... the portion is small and quite pricey but it's my way to unwind on a Friday. Vino Volo is also at Newark Liberty International (EWR) (Continental Terminal - I think). The one in PHL is just passed the TSA checkpoint, you can't miss it.