Not sure what time the service starts, but we set off from
Icmeler around 9-ish, I think. 2.50TL each on dolmus to Marmaris, but can't remember how much the bus to Datca was. Wasn't expensive though. When you get to the otogar, at the far end you will see an office with the Pamukkale sign above it, you go there to buy your ticket, and the buses are clearly marked which one is going where. There are staff walking around who will make sure you get on the right bus as well.