This paper proposes a storage system cost model for Infrastructure as a Service (IaaS) Cloud. The proposed cost model takes into account the virtualization environment, the storage system characteristics in addition to energy and QoS related parameters (Service Level Agreement and penalties). We show that those parameters are relevant and allow us to predict an accurate estimation of the overall cost of the IaaS infrastructure. We validate this cost model against real measures and we show less than 10% of error in most cases. Designers and administrators can use this cost model to perform optimization, load balancing, configuration and pricing of the Cloud infrastructure.