cost of taxis

Red line indicates Aurelian Wall. Area to which fixed fared from airports apply.
Source wikimedia

In Rome there are fixed prices for taxis to and from the airports to the city, written on the door of official taxis which stand in taxi ranks at the airport and elsewhere. Taxis from Fiumicino [FCO] have a reputation for sticking to the fixed price. Taxis from Ciampino [CIA] are apparently a bit tricky. The fixed prices are to and from the airports to places within the Aurelian Wall. Where most people stay.

Taxis can be called by phone and found at taxi ranks, but not flagged down in the street.

from a newspaper article about people complaining
that too many taxis are waiting for tourists,
not serving locals
This is a taxi rank outside St Peters.

The website numbeo, with general info on expat life, has taxi calculators for various places. It and other calculators can be found with a search for "[name of place] taxi calculator." Thus for Rome:

Use this for calculating taxi fares
https://www.numbeo.com/taxi-fare/in/RomeObviously traffic and waiting times will vary.