In recent years, cloud providers have introduced novel approaches for trading virtual machines. For example, Virtustream introduced so-called muVMs to charge cloud computing resources while other providers such as Google, Microsoft, or Amazon re-invented their marketspaces. Today, the market leader Amazon runs six marketspaces for trading virtual machines. Consumers can purchase bundles of virtual machines, which are called cloud-portfolios, from multiple marketspaces and providers. An industry-relevant field of research is to identify best practices and guidelines on how such optimal portfolios are created. In the paper at hand, a cost analysis of cloud portfolios is presented. Therefore, pricing data from Amazon was used as well as a real virtual machine utilization dataset from the Bitbrains datacenter. The results show that a cost optimum can only be reached if heterogeneous portfolios are created where virtual machines are purchased from different marketspaces. Additionally, the cost-benefit of migrating virtual machines to different marketplaces during runtime is presented. Such migrations are especially cost-effective for virtual machines of cloud-portfolios which run between 6 hours and 1 year. The paper further shows that most of the resources of virtual machines are never utilized by consumers, which represents a significant future potential for cost optimization. For the validation of the results, a second dataset of the Bitbrains datacenter was used, which contains utility data of virtual machines from a different domain of application.