But the larger your virtualized environment, the more you will need such experts. You might recoup their costs by needing fewer server admins, but you might not. After all, you also have a SAN to manage.

Another cost that can be hard to estimate is virtualization training, warns Prigge. The admin, for instance, will need to know how to reboot a virtual machine without restarting the whole host. "In some situations, the customer's inability to dedicate time to learn how to use a system suggests that sticking with physical servers with all of their limitations is actually a better course of action," he says.

Many but not all these costs can be inputted into good ROI calculators for server virtualization from VMware and Microsoft. These calculators can bring some clarity to a potential investment. Moreover, their predictions are often on the money with the actual ROI, says Wolf.

Virtualization's elusive ROI

If your reason to invest in virtualization is just about the numbers, you may be disappointed -- especially if you're a smaller business. Consultant Prigge figures that many smaller businesses that adopt virtualization pay 10 to 15 percent more when all is said and done.

But this extra cost, he says, "is justified by the increased capability to recover from a hardware failure. If you're not paying attention to business continuity gains, of which server redundancy is just one, you're sort of missing the boat," says Prigge. After all, virtualization's greatest benefit is flexibility and simplicity of business continuity for the entire datacenter -- not merely reduction in server boxes.

© IDG 2009