Calculating cost of network utilization

Hello, anyone have some kind of industry average “benchmark” cost for network traffic? Assuming, say, a WAN for a mid-sized company.

I’d like to be able to put a “price” or cost on our network traffic.

Example: a 500 Kb Word document is sent out as an e-mail attachment to 1,000 employees. How can I get a rough idea of the “cost” of this network traffic?

I know that there are so many variables based on network configuration, but some kind of industry average or benchmark is all I’m looking for.

Thanks!