In any of these assumptions, the application SurplusMeter, which is also totally free, you can very useful.
The tool It could not be simpler, you just have to select which day is the beginning of the month, the limit you have of data (or the one you want to put if you have any type of flat rate) and the connecting device what do you want to superviseAirPort, Ethernet, etc).
With these dates, you will have a summary of the data that are used up and down and some more stats.
Not a great app but fulfills its purpose and also at zero cost.
