Istanbul is a massive city split by the Bosphorus straight (?) into a European and Asian side. The euro side is where most of the tourist, nightlife and business stuff is. The asian side is a bit more like suburbia.
In my experience rent was more expensive in Istanbul than Sofia. Around 400+ Euro per month on the euro side. You'd expect cheaper on asian side.
Food costs were quite low when eating out, but very low when market shopping. At best, around 1USD per kilo of the best peaches on the planet. Like drinking sweetened peach juice. Cherries and grapes the same.
I used to live on the european side and commute to asia to do my market shopping. :D (Better produce).
Sorry for rambling.
Upsides of Istanbul: farmers street-market lifestyle - get incredible produce super cheap. Very happening city full of new experiences. Crossing the bosphorus on a ferry as part of your daily commute.
Downsides: crazy traffic, quite dirty (think HCMC), byzantine bureaucracy, not much english around if you're living like a Turk.
If you go to Istanbul, use my app to navigate the public transport:
https://itunes.apple.com/en/app/transportist/id644276437?mt=...
PM or email or whatever if you want a contact for a livable apartment block right near the centre in a gypsy slum.