How to monitor usage per subnet

[Steve Prescott]

I have a requirement to be able to report (on a monthly basis) on internet bandwidth usage by internal subnet.

I've tried searching for something to achieve this without much success as all the products I have come across look far to complex and include too much information - it will be an end-user who will be responsible for collating the data and billing the end-user.

To complicate the matter, the internal subnets may be duplicated but sit within different VRF's.

Any pointers or suggestions would be most welcome.

Thanks in advance,.

Steve

[Steve Prescott]

To simplify, VRF's are no longer in the equation.

I was thinking of connecting a small router into the core switch and spanning the port that connects to the firewall, then using NetFlow on the router to somehow provide data to some package on a PC? 

Am I on the right lines here?

Does anyone have any suggestions on software that will provide the data I need (i.e. total volume downloaded for current month by subnet?)