I looked into this in detail in the summer and whilst I am no expert, this is what I came up with, and used:
I got a PAYG Vodafone SIM to put in an unlocked smartphone.
I put £5.00 on it - (Make sure you turn mobile data off at this stage as it charges about £2 per day for mobile data in the UK).
I bought a "Big Bundle" voucher for £30 which I kept until just before I went out to Turkey.
I activated the £30 "Big Bundle" just before my trip, and turned mobile data back on.
All the minutes, texts and data allowance are available anywhere in the Vodafone "Europe" list, which includes Turkey. That will last for 30 days.
It worked a treat and it was especially good to have the mobile data, which was pretty impressive 4G whilst in Turkey.
After the 30 days it tries to auto renew, but cannot unless there are enough funds in the phone, so it just reverted to normal PAYG with the balance of my original £5 still available.
I made a diary note to top up with another £5 after 3 months and I sent a random text message so that the phone will continue on PAYG. It needs to be topped up and used at least once every 3 months to keep it alive.
Next year when I go back to Turkey I will put enough top-up in the phone to activate another "Big Bundle" - maybe less than £30 next time as that one had way more minutes, texts and data than I actually needed, and they do them for £10 and £20 with lower usage allowances.
They also have a very useful app which shows you exactly where you are with your usage of minutes, texts and data.
So, I hope that might help someone, and apologies if was a bit of a ramble.