Turkey has three mobile operators (per my knowledge), and all of them offer LTE data plans at reasonable price.
Beware of Sundays
Most of the mobile operator outlets are closed on Sundays. I landed in Istanbul on one Sunday morning, didn’t purchase SIM from the airport (assumed it’d be expensive here) and later found out that most shops are closed on Sundays. I almost gave up on getting the SIM on Sunday, before I ran into a place where I was able to purchase one (it was an exception).
Purchase SIM from the airport
Mobile operators have their own mini stores on airports. Most convenient is to purchase SIM directly from there as soon as you land. SIM card prices are standard, so you don’t have to worry about paying any surcharge at the airport.
Get plan with 3GB of data at 100 TRY per month
I assume your primary purpose is internet connectivity. 3GB a month is enough limit to cover your trip, and you can always top-up more GBs at reasonable price, should you run out of your limit.
I got a SIM from TurkCell, I assume other operators offer similar plans at similar rates. I hear TurkCell has most coverage out of all operators.
Do not throw away your card containing your SIM
The containing card might look like a useless piece of weight, but do not throw it away. It contains important information that can rescue you from some difficult situations.