I'm looking for a way to monitor and/or cap the monthly network transfer of an Ubuntu Server 12.04 process. I'm running a Plex Media Server, and while my current ISP is very lax on my bandwidth usage, I'm afraid that may be changing soon and want to be prepared.
I've looked into both NTM (http://netramon.sourceforge.net/eng/index.html) and Trickle (http://manpages.ubuntu.com/manpages/precise/man1/trickle.1.html) but NTM appears to need a GUI to run and doesn't monitor on a per process basis and Trickle doesn't appear to allow total transfer caps, only upload/download speed limitations. All of the posts I've seen on Ask Ubuntu are related to these two applications.
Is anyone aware of an application that will let me set a total monthly transfer limit, or any other way I could work this out?