International brands cost around $3-4 right now. The tax on the cigarettes has been increased twice by the government within the last two years. Buying cigarettes at the duty free shop at the airport would be smart.
Here is a website that tells you the cost of living in Istanbul: http://www.numbeo.com/cost-of-living/city_result.jsp?country=Turkey&city=Istanbul
(It may or may not be accurate)
And, here are the limits for shopping at the Duty Free Shop: http://www.istanbultrails.com/2008/03/duty-free-upon-arrival/