Taxi from airport to old city area near the Blue Mosque was $15 last December.
Some drivers don't speak English. At the airport, our driver was told where we were going by a supervisor (?).
We stayed in Istanbul for a night and it was well worth it. We walked to all the sites posted above from our hotel, the Four Seasons, which has great rates (for them) off season. The money we saved on tours for three pretty well paid for the room. A taxi to the bazaar was $6.