The ZTL (limited traffic areas) are permissions to enter into the city centre with bus.
The costs vary from city to city.
Here are some links of the major Italian cities to know the cost.
ROMA: www.atac.roma.it
FIRENZE: www.buspass.it
VENEZIA: www.asmvenezia.it
MANTOVA: www.mantovaparking.it
BOLOGNA: www.bolognaturismo.info
ASSISI: www.comune.assisi.pg.it
VERONA: www.amt.it
SIENA: www.sienaparcheggi.com
