Take a look at "Onavo" in the App store. It shrinks your data usage so you don't use as much data. As a bonus, it will also tell you how much data it shrinks for each installed and used app. Take a look at the screenshots on itunes and you can see if it's for you. It's free so it can't hurt.
As an aside, I had to disable my use as I think they were having server problems one day and I couldn't access the web. I couldn't figure it out as I had enough bars. I finally disabled Onavo, and then I was able to access the internet. I still think it's a good app to have ... when it works, it works well, and when it doesn't, you can disable it, then enable it at a later time. Even if you can only use it part of the month, it will still shrink some of your data use.