Buses run very often, and can be an interesting and enjoyable experience.
You can even catch a clown act on the local buses. Red or green buses travel a back route that takes you through the back side of the city, through tunnels cut into the mountains. They range between 10-20 pesos for destinations inside the city limits. Fares are posted in most major hotels and around town. Taxi drivers have been known to fudge on the fares.