myskipper: "he asked for more and claim it was the "service charge"".
Just to clarify for anyone who doesn't know the system: there really is a service charge for taxis departing the airport. It's 50B on top of the meter charge. Also, passengers are expected to pay the expressway tolls (various sums 20-50B, shown at the gate). Usually the driver asks for the money as you approach the toll station; if he doesn't, he will expect it at the end of the journey. I can understand the driver appearing nervous about raising the question, if the passenger shows no sign of understanding this and has no common language. On the other hand, anything running to hundreds of baht is a scam.
While we're here, here's my honest Bkk taxi story. I was in a taxi from the airport, at night, when the driver received a call. He stopped, turned around and found the bag the previous passenger had left (in the dark) on the floor in the back. We made a slight diversion and he passed the bag over to another taxi, I assume to be reunited with its owner.
And for balance, the dishonest one. This was a clocked meter, obviously running at way over the correct rate. Maybe half way between the airport and central Bkk it was indicating 40km, when the entire journey is less than 25. Luckily there were two of us and my companion was fluent in Thai, so that "meter" ride suddenly became a (reasonable) fixed fare.