Update:
from 1° OCtober 2023 City Tax is 6€, per night, per person, up to 10 nights, from 10 yo.
========================
from 1st September 2014, City Tax is 3,5 €, per night, per person, up to 10 nights, from 10 yo.
http://www.turismoroma.it/news/contributo-di-soggiorno
=======================
From January 2010, 2 euro each per night. up till 10 nights