At the airport there are car services that range anywhere from B700-1000 - either get them at the counter or with the folks roaming the halls when you exit with your luggage.

My first trip to Bkk in 1979, used Thai Limo (car service) at old DM airport for B450 when the taxi ride was B120. Have taken a taxi ever since and I make 2-3 trips a year.

A taxi to Sukhumvit will cost under B300.