[RobAley]

Bear in mind that most large scale public organisations primarily rely on either internal IT staff or external contractors (as opposed to the OS/application vendor) for user-facing support for both Windows and Linux. The scale & cost of that support is pretty much OS independent. Minimal training is required for front-line support of Linux for someone with experience of Windows support, something akin to the training required for a new version of Windows.

Central provisioning, deployment and higher level support costs are centralised and do not scale as fast as user facing support. These areas are typically contracted out to (or supported by) the OS/app vendors, and such cost is a smaller part of the deployment/maintenance costs.

Large enterprises perhaps do not work in this manor and have often have direct per-seat support from vendors, but public organisation are generally organised as I outline above (at least over here in Europe).