The device or one-time cost of registering a SIM?
Are there large chunks of the population that just use wifi-only phones?
Are there large chunks of the population that just use wifi-only phones?
There is no wifi only phone or wifi network, if you really mean wifi. There are "only data" plans but for that you also need to register the IMEI.
Last time I visited Turkie, I didn't had any trouble using the phone (except eye-watering 3G tariffs and somewhat spotty reception out of the main areas), bu the moment I tried to use a WiFi network in the mall it requested a SMS confirmation.
I've heard some people clone IMEIs from the old phone when getting a new phone from abroad and that it does reduce the cost significantly.