I recently found myself considering moving one of our servers to the Rackspace Cloud. I needed to get an estimate of the cost for bandwidth usage.
I remembered I had installed vnstat on that server when it was deployed.  This made the bandwidth calculation completely simple:

Just have vnstat give me the bandwidth sliced by Month:

$ vnstat -m

 eth0  /  monthly

   month         rx      |      tx      |   total
-------------------------+--------------+--------------------------------------
  Aug '09       7.00 GB  |     8.48 GB  |    15.48 GB   %::
  Sep '09      14.98 GB  |    15.75 GB  |    30.72 GB   %%%::::
  Oct '09      31.70 GB  |    14.79 GB  |    46.50 GB   %%%%%%%%:::
  Nov '09      45.46 GB  |    14.64 GB  |    60.10 GB   %%%%%%%%%%%:::
  Dec '09      73.71 GB  |    15.40 GB  |    89.11 GB   %%%%%%%%%%%%%%%%%%::::
  Jan '10      66.25 GB  |    16.89 GB  |    83.14 GB   %%%%%%%%%%%%%%%%::::
-------------------------+--------------+--------------------------------------
 estimated     94.97 GB  |    24.21 GB  |   119.18 GB

The install of vnstat on Ubuntu can be done with apt-get:

sudo apt-get install vnstat

Remember, once installed to initiate you vnstat DB.  Just execute vnstat and it will tell you how.   vnstat -u -i eth0 in my case

Then check the man page for the command to see all the different ways you will be able to slice and dice your network data