Getting quality food for a decent price at an airport while traveling is challenging.
A best value I discovered is to look for a Mexican cantina of some type and then order a "burrito bowl," which is all the innards of a burrito in a bowl, without the exterior corn tortilla wrap (sometimes called a "naked" burrito).
Unfettered from the finite volume limitation of the tortilla, you can request extra beans and vegetables in your bowl for no extra charge. You'll still pay $8 bucks, but get a lot more useful food that way.
Often if you get a veggie burrito, added guacamole may be free, depending on the vendor. Don't be afraid to ask and preamble your question with a universally persuasive phrase like: "Is it cool to ask you for extra black beans?" This kind of wording is respectful of the server and gives them the decision making authority. Naturally, no one wants to be perceived as "uncool," so usually a server will accommodate such a request and pile on the black beans.
"Is that cool" is a remarkably persuasive phrase, for reasons I don't quite understand. Almost every time I have begun a request with "is it cool if...", I get compliance from the other person. Try using it and see what kind of results you get, then leave a comment here with your observations.