A taxi from Dalaman Airport to Dalyan may cost very much for you since those taxis are very expensive. 5 kms cost a min of 15 dollars. Thus, from Dalaman Airport to Dalyan would cost you about 75 dollars.
Instead I would suggest travelling by bus. There are buses from Istanbul to Mugla-Ortaca and sometimes some bus companies have services or direct buses to Dalyan.
I would suggest Kamil Koc (www.kamilkoc.com.tr for online ticket sales) for a clean way for such a travel. almost 10 years ago