Pricing structures for the cloud are based on a various factors ranging from storage space needed to clock cycles used to monthly traffic allotments, and it does not end here. Sometimes there are additional charges hidden deep within service-level agreements (SLAs). To arrive at a total pricing for a cloud service, user organizations must take note of individual service elements that a provider bills for and how these are calculated. Does the provider, for instance, bill based on within server traffic, storage space needed, server CPU time or a combination of these factors along with other elements?
http://www.cloudtweaks.com/2012/04/cloud-computing-determining-the-cost-of-the-cloud-part-3