I discovered recently a cheap way to travel to and from Santiago Airport.
There is a bus named CENTROPUERTO. From the city you can get to this bus with Metro Linea 1, to the Station LOS HEROES (where the bus starts).
The cost is 1900 pesos (approx 2.70 US$) one way, or 3400 pesos (approx 5 US$) both ways. You pay the driver in pesos. The return ticked is valid for 90 days.
The only problem is that, although the bus lets you in with big suitcases, the metro does not. I have heard that you have to have a smaller suitcase than the carry on, but I have seen people in the Metro with one carryon suitcase.