Enchiladas and crepes are especially common here, often with potatoes. The best meal cost one dollar and was bought on the sidewalk from a crouching elderly woman (for all the talk about "street food," often "sidewalk food" is where it's at). It was potato, nopalitos (cactus), finely ground white cheese, and a potent chile sauce on top of a fried blue corn tortilla.
At the local Arabic-Mexican restaurant, ten chalupas can be had for $2.10.
In Mexico never eat until you are full, because you will likely encounter something even better along your way. What is hard is not finding the food but rather enforcing the optimal stopping rule.
If you are trying to argue that Mexico is a "normal" country, this city is your Exhibit A.
The much-vaunted decline in the Mexican birth rate is somehow not in evidence here; perhaps that is an artifact of who visits the Christmas displays. Plenty of police are out with guns, as a signal to deter a potential drug gang invasion.
As Yana notes, on the streets you will see many examples of perfect competition.