Many airports offer fixed Flat Rates to the city center — check the official signage.
Flat rates were introduced at major airports largely in response to decades of complaints about overcharging. New York City, for example, instituted its famous JFK-to-Manhattan flat fare in 1990 after years of tourists being taken on circuitous "scenic routes." London Heathrow, Paris Charles de Gaulle, and Rome Fiumicino followed suit at various points, with Rome adopting a fixed €50 fare to the historic center in 2012 after a particularly bad stretch of tourist scams during the Vatican Jubilee years. The lesson: look for posted official rates on signs near the taxi stand or on the airport's website before getting in. If a driver quotes a different price, that's a red flag.