It costs a lot more than a new PC to upgrade thousands of PCs. Imaging, deployment, backup/restore processes for the end users is just the beginning. Upgrading dozens, hundreds, or thousands of individual customized applications to be compatible with Windows 7 is an absolute nightmare. I know all about this just from upgrading my relatively small workplace from XP to 7. It was a fight just to get core, mission critical apps to work with IE 9; 10 and 11 are out of the question. Lots of cash to vendors and app support folks, lots of cash to deployment specialists, lots of overtime. Adds up to a LOT of money.
By the way: $9 million over 680,000 PCs is $13 per PC. That's less than we paid per PC to have a contractor come in and physically install new machines at desks, and completely ignores the cost of OS licensing, hardware, support, and the thousands and thousands of man hours the IT department spent with associated tasks.