There's more then only one side to the coin...
clouds might seem hip today, but guess what? We all are using them daily! There are multiple kinds of clouds, internal, external, and there are multiple roles: Provider, user, etc.
Google uses a massive cloud internally FOR US, so that we can search the internet (this is one example of an internal cloud).
Amazon provides an external cloud for us to consume.
So, I guess, reducing the measurement of the usage of clouds to "dollars spend" or "dollars saved" for a single application, does not provide the right thinking.
An aspect, many do not see, when talking about clouds is: You do not need to spend TIME in the ordering and set-up process of the IT equipment, it's already there.
You do not waste CPU cycles, as with all big numbers, they do measure better.
So, if you quickly need an IT-piece, but only for a short time, then a publicly available cloud is your thing. So, no need to worry about the "removal" of the equipment, once the job is done. That will be used by someone else...
Yes, it might not be the right answer to all your problems, but same as with the power industry, we all at home do no longer produce our own power, we BUY it from the "grid" (to use an older term), and the producers of power can scale according to needs, and be more efficient overall. If you would only need power from 8:00 pm to 11:00 pm, then you might be better off, buying your own "power-plant" and running it yourself, but you need to remember, that "taking care" of that also adds to the overall equation. Many people tend to forget the "time" and "laziness" factor, which we pay premiums for, for NOT BEING forced to take care ourselves. Same applöies to insurances: You pay for the security, but you never know, if it really pays off (Ever done a ROI calculation of your health-care-insurance? I doubt, you like the numbers in many of your individual cases!).
So, as stated, you need to think of more then only shifting usage from 10% to 17% by doing virtualization, there's more to the overall equation. (and, did I mention, that many of the newly created public clouds will be located CLOSER to the power plants, so that CO2-emissions and LOSS of power during transportation can be reduced?)