I would suggest booking a private shuttle or a private car service with one of the travel companies, instead of dealing with a cab. Why? Because cabs are not on Tripadvisor, but these travel companies are. They are more motivated to give you a great service. They usually cost around 15-20Euro (per person) for shuttle transfers and a lot more for private cars. I went for a Istanbul day tour with True Blue Tours recently (as a guest), and we were traveling with their shuttle buses. They are really comfortable, and the personnel was really nice to us. Seems like their shuttle service prices, and private cars are relatively cheaper. Here is their Tripadvisor rating. There are many more reputable companies out there. Shop around and book a shuttle from and to the airport for your convenience.
If you want to compare it to taxi/cab rates, then you can check http://taksiyle.com/ (if you use the English version, the total cost may seem to be hidden behind the POI tab area)