Another possibility to try is to request a "Collectivo" ... a shared Taxi that will drop each person off at their individual locations. Ask for this just outside the doors from the Arrivals Lobby to the Taxi Stand. There will be a guy coordinating all the Taxis... ask him, or go to the Taxi booth just to the right as you come out the door. If you are not in a group already you may need to wait a little as they will usually want at least 3 people. (you can speed things up by asking other arriving tourists (backpackers?) who might be interested and see if they want to share the ride) This only works to the city that I know of. The Collectivo cost $12 per person when I was there end of last year.
Returning to the airport has gotten much more expensive over the last few years. What I do is ask various Taxi drivers during my stay (that seem to be good drivers & have good cars w/ A/C) how much they would charge. Some will be willing to bargain a little, but not too much less than the standard $25 fare anymore. Most Taxi Drivers will have cards so you can call them to pre-arrange rides & pickup times.

Ron