Minimum 200 TL for food and transit you need in Istanbul for 2 people. You could do the conversion in Euro. Less than that, yes you would get by but you won't enjoy as much.
well average food prices for a meal is between 10-20 lira(McDonalds,kebab,doner)
i recommended you to buy Istanbul card for bus and metro it will cost you 10 lira and every ticket will cost 2 lira museums fare are 10 to 50 lira. if you have more question i can happily answer them. Btw these prices for Istanbul. ( I'm sorry for my bad english :) )
2/3rd of what you will need to spend in any other European Country.
500tl (each)
Including transportation, food and tourist attractions