An orange taxi would cost around 350 TRY (roughly £10*). The distance from Istanbul Airport to Sultanahmet Square is about 50km and would take roughly an hour (depending on traffic).
There are three types of taxis in Istanbul, each offering a different level of service. The orange "C" type taxis are small, economical vehicles, such as the Fiat Aegea or Renault Clio. These are a great option if you're looking for a budget-friendly ride. The blue "D" type taxis are medium-value cars that offer more comfort, with options like the Opel Insignia. For a more luxurious experience, you can opt for the black "E" type taxis, which provide premium services and vehicles.
“D” type taxis are roughly 15-20% more expensive than “C” types, and “E” type taxis can be as much as 80% more expensive.
But remember, traffic can affect the cost. If you get stuck in one of Istanbul’s infamous rush-hour traffic jams, you might end up paying more than the estimated fare.
*Please check the current exchange rates as they may have changed significantly since writing.