My wife and I were in Istanbul last January. At the Istanbul Ataturk Airport there were 3 stores who sells simcard.
The prices are almost same 80 Turkish Lira for just simcard. But they are overcrowded.
We finally rent a mobile hotspot from iugo gadgets for rent at the arrival terminal.
We paid total of 40 Euros for 7 days. The mobile hotspot had unlimited data.
It was cheap comparing to other simcards..
We dropped the device back to the store before our flight