Several regrettable episodes happened during the past years such as unlicensed private drivers offering expensive lifts or even regular taxi drivers both asking unacceptable fares for the itinerary from airports to city center forced the Council few years ago to find an agreement with Taxi unions to set fixed fares.
Actually from Fiumicino FCO airport to the city area inside the Aurelian Wall the fixed fee is €48 each way, including luggage and extra charges.
From Ciampino CIA airport to the city area inside the Aurelian Wall the fixed fee is €30 each way, including luggage and extra charges.
You can easily read the fares stuck on each front door of regular cabs.
Other common destinations are regulated by fixed fares, you can visit the following pdf document published by one of the most famous taxi coop and check all taxi fixed and by meter fares including the Aurelian Wall map.