Great question — answering as the UphillTech team:
30 euros is possible, but you'll have to search for it. Air conditioning and big windows are a bit specific, which means you'll simply have to search longer. Try http://www.hotelscombined.com/ for getting, often, the best rates on hotels. Try http://www.airbnb.com/ if you're ok with staying in someone spare bedroom, or garden bungalow. Typically, the latter will give you much better value for money, particularly if you're ok with not staying in the tourist center of Istanbul (which is Sultanahmet).
Full answer by MastaBaba: https://travel.stackexchange.com/a/8480 (CC BY-SA 3.0).
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