Just having gotten back from Singapore last night, let me offer another option to the airport. Your flight is early enough that you can take the MRT from Orchard Station to the airport. It will wind up being less than 2SGD once you get your 1SGD deposit back...
I paid 17.35SGD for a taxi to the Marina area on Friday night, but you wouldn't be paying the surcharges since you're leaving before they kick in.