Easy in Android
Tracking and limiting your data usage is easy on an Android device. There are a number of options available to stop yourself from going over the data limit.
The simplest solution is to activate two options under the Data Usage menu in your phone's Settings app: Limit Mobile Data Usage and Alert Me About Data Usage. Toggle these two switches to on and your phone will turn off cell data once you reach a certain point, as well as warn you when you start getting close. Adjusting those thresholds is as simple as dragging a little line up or down.