Data Roaming is the setting you're looking for. Turn it off. It's under General > Network.
They recently added another setting to shut off Cellular Data altogether, but since you will be roaming regardless of where you are in Europe, you should just shut off Data Roaming. That is the setting that has always existed for that purpose and so there's no real reason to use the Cellular Data setting.
As soon as you get on the airplane, turn off Data Roaming and turn on Airplane Mode. As soon as you get off the airplane, turn off Airplane Mode. Reset your usage statistics in General > Usage. Only turn Data Roaming back on when you need to use 3G data (looking something up on-the-go, no WiFi, getting directions, checking GPS/Google Maps if you are lost). Turn it back off as soon as you are done. You will get a feel for how much data one of those acts uses by checking the usage statistic.
I was in Italy for 5 weeks, and I bought the 100MB package. I had WiFi in my hotel, but I used cellular data occasionally for Google Maps, quick searches/lookups, and I ended up using just about 85MB in one month (4 weeks). As long as you're not streaming YouTube/Pandora or using your phone as a turn-by-turn GPS/nav device, you should be fine with 50MB for a week or two, maybe more if you're vigilant. 50MB of data really should be enough - but it depends on how long you're going for and what kind of user you are.
Just monitor the usage statistics (don't forget to reset it when you arrive), make sure Data Roaming is off unless you need it, and enjoy Europe.
