Recently I went into price models for cloud computing and this is an interesting figure from Amazon.
The picture came from the Amazon developer guide and is more or less a good summary. In few words you pay for CPUs, data coming in and out amazon and permanent storage.
What is not clearly shown in the picture is the costs for the amazon services. In other words every time you perform a change or monitor in the infrastructure with external tools you must pay something because you are using the web services of amazon. If you have a lot of datas or you are using many virtual machine this cost is negligible but if you plan a close monitoring to your infrastructure you should be prepare to add this extra costs.