Some virtualization vendors have an online ROI/TCO calculator that prospects and customers can use to justify the purchase of their hyperivsors: VMware has one, Parallels has one.
Microsoft, which is preparing to release Hyper-V 1.0, now has one as well: the Microsoft Integrated Virtualization ROI Tool.
Compared with the others above, this calculator requires an impressive profiling of the company's current situation for three different areas: production server, development and test lab, and desktop virtualization.
From that data and for all these scenario, the tool provides a very deep analysis of the TCO, the entity of the investments (up to 3 years) and the ROI.
Another interesting aspect is the comparative calculation between Microsoft products (both Hyper-V and Virtual Server) and competition, with consideration for all the software licensing, the storage space cost, the third party backup software and more.
But probably the biggest selling point of this product, which is free of charge, is that the model, the methodology and the statistical data to define the average values is provided by IDC.