You have the option of either a flat fee agreed up front or a metered taxi trip. A metered taxi trip is always going to be the cheapest. Thai taxi meters have a flagfall of 35 baht and operate mainly on distance -
Public taxi stands are located on level 1 (one level down from the Arrivals level). If you select a metered taxi the driver may ask you to provide the toll fees en route - the booths have plenty of change.
However, when I went to level 1:
I had to queue then was given a lane number to meet my assigned taxi:
and the taxi wasn't metered (I was instead given a fixed fare). Am I supposed to argue with the taxi driver, do the queue again, or grab a metered taxi somewhere else?