I have the 2gb plan. This has worked fine for a year until yesterday when our wifi went down and I turned off wifi so I could get my email. I forgot to turn wifi back on...big mistake.
Understand that I am only 15 days into my 2 gb. While i am asleep at 2:41am a message came saying I and 20% data remaining. At 3:05am, 9% remaining and at 3:37am 1%. In other words, while I and my iPad were asleep, it used 400 mb of data in one hour.
After turning wifi back on and renewing my data plan, I looked at what was running and closed everything out. The problem is that I really don't know what app was draining my data. The moral to the story is close your apps and turn it off (not just sleep) especially when not in wifi.