I'm visiting Italy right now and have a 3gs from att. I jailbroke and unlocked and then went to a Vodafone store and got a new sim there. It cost me 20 euros total, 5 for the card and then 15 euro credit on the card. Plus they had some promotion where I got 6 euro credit free, so I ended up with 21 euro credit for only paying 20. I don't remember how much calls are because I wasn't too concerned with that, just the data. The lady signed me up for the iPhone data pack (which she said would be on in one hour but it took one day to receive the text saying it was activated). The data costs 3 euros a week and she said it's unlimited but according to a text I got from Vodafone it says it's 500mb a week.
Hope this helps, if you have any other questions just let me know!