With mouthwatering cheap eats well worth detouring for, La Terraza Amarilla is a gem: you choose between four styles of mains (tacos, tortas etc) and one of seven different fillings, and in seconds your plastic plate arrives. Each menu item is the recipe of the chef-owner and his wife, who will lovingly detail each option for you given the chance.
With the different combinations you could probably eat here every night for a month without having the same item twice.