The number floated around for XP continued support is $200 per machine per year. Redhat, which is one of the few companies that supports a Linux distro for as long as Microsoft supports their products, charges $49/year for the equivalent of Windows Update, and doesn't have any information whatsoever on extended support. So, for the average customer which wants their desktops to have the same lifespan as XP in order to minimize training and other associated costs, ending up with a year or two of Extended Support while you finish your transition is still cheaper than any of the Linux providers.