Charging / cost for Internet usage

Hello I have a client who has a cyberoam firewall device which is a fairly decent device.

I can get reporting on how much data is used between each department so the cost of the diginet lines can be allocated to the various departments.

My questions are:

1. is allocating cost on data usage the best practice, as department X may use the most bandwidth but not necessarily the most data.

2. Is head count a fairer way of splitting the costs.

any thoughts and ideas welcome

thanks

Frank