I suppose it's depends on what type of phone you want.....If you want a Samsung Galaxy S3 or Iphone then the difference in price UK/Turkey is enormous.
Price difference for Galaxy was £484 in UK v 1999tl, about £730, in Turkey.
100tl/£35 to register with tax office equals a saving of over £200....Simples...
If you want a basic phone by it in Turkey.