Network Disk Space Cost

I am trying to determine a formula for the cost per megabyte per month of disk space on the network. We are getting ready to implement a quota server on our network, and would like to internally charge departments who go over their quota.

Thoughts include determining hardware cost, maintenance contract cost (incl. backup), backup hours cost, and coming up with a monthly cost from there.

Has anybody done this? Any white papers on it? I didn’t see any in my search.

Thanks!
Ed Roden