I will answer in honest. All of gsm compinies are liar and thief in Turkey. Watch out your every steps sign the contrat or whatever you sign. Take help from someone for translate.
We have 3 big gsm company. Here is price list of companies but in Turkish :( Vodafone Avea Turkcell
. You can understand numbers and prices. And when you buy a new sim-card, they want to look your passport. And you will pay circa 30 Turkish Liras (last days of every months compinies have discount on this price, you can bargain)
I hope, my advice is useful and you will handle it.