Cloud computing is an emerging computing paradigm that replaces computing as a personal asset with computing as a public service. As such, it offers all the advantages of a public utility system, in terms of economy of scale, flexibility, and convenience, but it poses major problems including the loss of availability. In this article, the authors define and refine a taxonomy of basic security requirements suitable for all contexts and systems; then the resulted hierarchical model is used to create a new approach to quantifying the availability of it systems. This new measure is inspired from the mean failure cost (MFC). Measure and called availability mean failure cost (MFCa) is the average monetary value of loss per unit of time of use of each participant. This metric gives us a more accurate estimate, clear refinement, and useful interpretation for availability-related decision making using MFCa. How this metric can be used to analyze cloud computing as a business model is something to be explored.