Prices in the domestic departures area at IST are significantly cheaper than international departures.
Prices for those flying internationally are not intended to be remotely comparable to what you would pay in Istanbul or any Turkish town or city. IST as a new airport is far from any real town if you do not have a car - it is very time consuming for intl transfer pax to access food/drink sold by independent vendors who are far from the airport. You may see many different branded shops airside at IST - but a single company will operate many brands under licence and co-ordinate prices (ie keep prices high) amongst each of those branded outlets. The pricing policy is about maximising profit from a captive audience. Non-managerial staff at IST are fully aware that many pax are surprised at the high retail prices - they have conversations with pax about this on a regular basis.
Thank you for the comprehensive explanation.