There's a small storefront just north of Chicago Ave on Marshfield that serves consistently excellent, authentic, cheap mexican food. It's called Mr. Tacos, and despite the name, seems to be run by people who know what they're doing and make everything there from scratch. They say that gorditas are their specialty, but their breakfast food, like the chilaquiles, is excellent, as is any other dish I've had there, and I've eaten there at least 20 times.
Gorditas are about a buck fifty, and three is a full meal. I really like the chile pasada (chiles, eggs, and cheese) and the carne asada (carne asada, cheese and beans) gorditas. Not a fancy place by any means, but excellent, cheap mexican grub. It's a good cheaper option to La Pasadita in the area (it doesn't have the late night hours though).
Mr. Taco's Restaurant
810 N Marshfield Ave, Chicago, IL 60622