Is the per storage unit the only metric used to derive the internal transfer "cost"? Or is it the only metric?
Hardware, backup hardware, floor space, cooling, network, external network, off site backup, transfer costs of media, depreciation, staff, software, software development, etc.
Do you provide separate funding from your cost center for the desktop, portable, phone, fax, printers, etc? Pay for per page printing in a tiered manner (desktop, group printer, black and white at the central facility, color, binding, etc.)?
What labor market are you in? Done any corporate downsizing so that fixed costs have stayed constant where usage has declined?
What I'm trying to understand is just what the per unit of storage cost has to cover.
What starts out as a simple question needs lots more background before you are able to compare apples to apples. Most IT groups don't make a profit as such but try to estimate for budget purposes whet you will need 18 months in advance of actual usage. Try it sometime and you'll understand that plucking a number is probably as good a method if done by the right person with an understanding of the organization and what it will be trying to do over the next year or so.