Hello,
Here is the info on how I setup Chargeback for storage at NASA-JPL.
The original equation had a base charge of $50/GB. This included clustered storage that was snapmirrored to a different building on JPL property.
I had worked the equation through trial and error to end up with
monthly charge = U * 50 / (log ( (U^2/1000) (U^4/200000000) + 10 ))
where U is the usage in GB.
If you plot that out, you can see a decreasing cost per gig overall.
That equation was a little confusing as the cost per gig was contained within the function.
The algorithm was reworked (down to $25/GB base) to break out the Cost per Gig, it became
Cost per Gig =25*(1.3348-LOG((U+100)/10)/3)^2
where U is the total usage in GB.
Once you have the cost per gig, monthly charge is,
Total monthly charge = U * Cost per Gig
Here is the real trick, you need to create a spreadsheet or some other method that will total your customers and all of their monthly charges to result in a Monthly Customer Total (MCT). Multiply 12* CMT to get an estimate of the the Annual Customer Total (ACT). Now you have to make sure you can afford all of the costs of supporting your storage infrastructure within that ACT number. Be sure to combine the costs for your systems, replacement hardware, the power, A/C, network, backup and system administration costs to get the Annual Total Cost of Ownership (ATCO). If ATCO is greater than ACT, then you either need to charge more, or figure out a way to reduce your ATCO.
It takes some trial and error, but after a few attempts at closing the gap between ATCO and ACT, you will get a better feel for that base ($25/GB ?) storage charge unit.
Customers will appreciate the Cost per Gig equation as it shows them economies of scale.
I hope that helps you balance your books.
Best,
Joe Bishop