Viva El Taco is a small Mexican restaurant that's open 24/7. It's not a very good-looking place, but they do have good food. You can either go in and sit down or drive through. The speaker has been broken for years, though, so don't expect to understand what they say. Just order what you want and drive to the window.
Everything on the menu is very affordable. You can easily get a meal for under $5. I highly recommend their breakfast burritos; they're great for breakfast or a late-night snack.