For the last couple years the government has had a big problem with the drivers of the buses and the security of the passengers. ESPECIALLY IF YOU ARE OLDER and trying to get on and off - they do not stop necessarily to let you off - so every month a dozen or more people DIE getting on and off the buses or being hit by them! So WATCH OUT when you cross the streets! and watch out getting on and off the buses! There are tons of them and a great cheap way to get around but you need to be careful !
The most inexpensive way to get there is by bus. They have great busses in Mexico, air conditioned, in route movies, restroom, and they make stops for food etc. It is also a great way to see the country. You san see a lot more at ground level than you can at 30,000 feet in the air.
Busses by far are the best way to go. Taxi's are O.K. too but negotiate before getting into the cab.