This is a bit of a late reply, but if you're running iOS 7 you can go in Settings > Cellular and then check the data usage for each app. This could help you narrow down the app(s) that are eating up your data. Some people think that Rogers is scamming them, but often it's actually a glitch in an app that has it consuming a whole lot of data. I've seen that happening with Skype, for example. So if you find an app that's used a lot of data even though it shouldn't, you could turn cellular data off for that specific app.
I've found with the iPhones it consumes quite a bit of data just leaving the data on all the time. Like my mom never uses her data and just cause we left it on all the time she used about over 90 megs.