Hi, there is a website that explains how to buy a prepaid SIM card at cost price:
www.saveonroaming.net
An insider trick: you buy a TIM prepaid SIM card for 5 Euros and then go to 3 and ask for number portability. You will get for 39 Euros an LG U300 (instead of 169). And then activate the option "Internazionale senza limiti" as said before.
Otherwise, a call costs you 25 eurocents/min.
More infos about Italy can be found
here