Thanks ISTFlyer. SAW is probably a bit too far from the city for me to spend the night.
Does anyone know what the going rate for taxis from the new airport to central Istanbul (say, Taksim) is? I'm trying to weigh up the cost of booking a private transfer online versus a taxi. Private transfers from the new airport to Taksim are around 50USD (~44km). A taxi from the old airport to Taksim would have been around 10USD from memory (~22km).