Istanbul Ataturk Airport is 18 kms to old city. This is 15--30 mins. driving distance depend on the daily traffic. Celal Sultan Hotel offers Limousine services with magnificent view of Marmara Sea along the road to the old city where the Hotel located. 
Ataturk Airport Transfer One way : 30 Euros Both way : 55 Euros Sabiha Gokcen Airport Transfer One way : 70 Euros Both way : 130 Euros prices difference +10 euros between 00:00--06:00