Dzebabyxo,
Hmm, I guess that you know about going to Settings > Cellular and scrolling down to CELLULAR DATA and looking at the apps using your data.
One thing that might help is to Reset Statistics (at the very bottom) to coincide with your billing period. Then monitor it through-out the month and when it approaches your limit, turn off Cellular data entirely and only use WiFi. I actually do that myself and it works reasonably well.
That's all I got. Good luck.