It's an express coach service between Istanbul and Fethiye / Kas, not dolmus. Unless of course you want to take about 3 or 4 days to do the journey. :wink:
The coach journey from Istanbul to Fethiye takes about 12 - 14 hours depending on the departure time and operator. It's a further 1 and half to 2 hours from Fethiye to Kas. Prices start at around 40 TL.
You can get flights from Istanbul to Dalaman starting at 35 TL but that'll depend on a number of factors. You've also got to add the price of transfer from Dalaman to Fethiye / Kas into the equation.
If you're going to Kas you could also consider Istanbul to Antalya and then the local bus to Kas. I've done Istanbul to Antalya overnight many times - cheap, comfortable and a chance to practise your Turkiah on the other travellers!