Estimating cloud services costs in advance

1) Down-time. The reason you go off premise is to out-source the worry and responsibility. What does it cost you (in money, and time) when someone goes wrong but your IT manager doesn't actually own the infrastructure to fix it? The cloud provider has many customers, your IT manager should have one - the business.

2) Ramp up. Has your IT staff or personnel used this service before? Are they comfortable with the tools or have they used something similar? Doubtful, as stable vertically integrated services are just now appearing.

3) Migration. Sometimes your migration path isn't as straight forward as moving your applications into VMs and hosting them on a cloud platform. Sometimes getting rid of an on-premise box means the difference between Microsoft Exchange 2010 and G-mail. Besides the cost differences between this example, all the meetings and coordination to make a switch are somewhat invisible and I'd say that's worse.

Ultimately, someone who is responsible for running the technical infrastructure of a business needs to understand what their primary problem is, how their actions relate to the bottom line of the organization, and how/when/why they would leverage a cloud platform. Only at that point would I say looking at CPU/RAM costs matter.