If you're in the mood for good, cheap Mexican food, you want to make sure you hit up Lolitas down in San Diego. There are several locations around SD, but for the most part -- you'll enjoy the same experience at each spot. You probably haven't heard of Lolitas on anything like the Travel Channel or the Food Network, because this is truly a local's favorite. Only people from SD know about Lolitas.
I've gone to Lolitas pretty much each time I've gone down to SD. No trip is complete without it. Granted, I haven't tried all of the food items, the main thing you'll be set with is either the Carne Asada Fries or a California Burrito (Carne Asada fries in a tortilla.) I've also had an order of their rolled tacos as well as their shrimp quesedea, both of which are very good.
Depending on where you go though, the place gets packed. They're calling out orders in the 30's and you're receipt sadly says 65. So one tip would be to call in before you get there, they gladly will get started on your stuff.