There doesn't seem to be a very reliable source of bus-route information with maps of the the areas served. For example, we gambled when we arrived at the airport with two heavy suitcases that anything marked TUNEL would get us to an area close to our apartment. We were right.
In order to figure out more than just our common routes, today, we just got on one and noted that it was a 2xx, which I think means a basic route #2 but going through a specific, but unknown to us, streets. Then again, it could mean anything. The view was nice at the end as we had gone up to the edge of the mountains.
The price to board a bus is 7.5 pesos (55 cents), although no transfers are given. Still, for the cost of a month of my truck insurance alone, as a couple, we can take 45 round trip journeys. That is more than one outing per day. This is not to mention the costs saved on gas and maintenance fees.