Not only does it give you the amount of data you have downloaded or uploaded so far, it also estimates the amount you will consume by the end of the month, if you keep the same consumption rates, based on some methods in statistics.
vnstat is available in the official Ubuntu repositories. You can get it by running
sudo apt-get install vnstatIt is an extremely light package.
Using it is not a big deal and I am pretty sure you will not find any trouble getting familiar with it. You can check your data consumption by simply running
However, you can always check its manual for more advanced features by running
Here is an example of the the output you get by running vnstat. ppp0 is usually the mobile broadband interface.