There's a fixed fare scheme, under which it should cost 19 Euro between the
airport and Beverello - although you'll often be asked for a little more!
The driver is meant to give you a receipt, something like this one... but
do be sure to ask to be charged that way BEFORE the meter is turned on.
When the roads are quiet, the metered fare may well be cheaper, however
Naples is famous for traffic jams - and you may well prefer knowing in
advance what the ride is going to cost?