I’m a big fan of the Mexico City subway system, which is super cheap, super fast, and super easy to navigate once you get the hang of it. It’s also remarkably comprehensive. This trip we managed to avoid paying for a cab, and only took the bus once—when we traveled from our van (which we left parked on the far outskirts of the city) to a metro that would take us to our hotel. However, from the sound of it, the bus system could be another viable budget travel option. Don Cuevas includes some useful info in a recent post.