No travel planning is complete without a budget analysis of how much the trip will actually cost you. This list is based on the three day itinerary for Istanbul that you can find here. The prices provided here are from 2015 and are subject to change depending on the time you wish to travel on, so there is a field for buffer cash i.e. an amount that you should take just in case the prices have risen or exchange rate is less in your favor. I hope this provides you with a basic idea of how much cash or credit amount you need to travel with. One more point to add here is that most places in Istanbul take credit cards so if you wish to travel light on cash, you can definitely rely on your card.