Why would Apple charge you for "data usage"? That seems more like a carrier issue. Furthermore, Apple allows for a certain amount of free iCloud "storage" space, and if you approach or reach that limit you will get a low storage space error and will more than likely be unable to backup to iCloud. Having said that, if you purchased additional iCloud storage space or someone with your iCloud credentials purchased additional iCloud storage space then yes, you will be charged. To rectify the problem, call Apple.
https://forums.imore.com/e?link=http...token=8fc8MCul