According to Flyertalkers on this board, you should just suck it up and pay the overcharge.
Beats getting hacked to death.
Maybe better to just note the taxi registration, look left/right just below the windows, two letters (Thai and English equivalents) and four numbers, or look at the dispatch form, and report it?
http://www.flyertalk.com/forum/archive/t-1466707.html
There are countless web and mobile apps. which allow one to estimate the fare.
This one indicates 241 baht, we know we must be exact here, for Suvarnabhumi to the Fenix (320 Silom Road).
http://bkktaximeter.appspot.com/en
http://aroimakmak.com/2013/04/bangkok-taxi-meter/
https://play.google.com/store/apps/d...?id=tt.m&hl=en
There have been some recent public discussions, May, 2013 re: raising taxi rates, be it flag-fall, distance or traffic jam, but any actions were shelved for three months.
http://www.thaivisa.com/forum/topic/...ng-cost-rises/