A taxi to the old Ataturk from say Beşiktaş which is where some decent tourist hotels are, and a nice location, near Dolmabahçe Palace was dirt cheap - last time I think it was less than 20e
and if one was on the early flights out in the morning (First BA to LHR) then there was no traffic and it took about 15-20 mins (taxi's at 130kmph lol)
50e is like ouch!