The French Gendarmerie (national police force) has been gradually moving its software infrastructure to open source over the past several years, starting with the adoption of OpenOffice.org on Windows, and now moving to Ubuntu for more and more of its desktop computers.
While some governmental migrations have surprisingly cost more than proprietary software (due largely to training expenses and ancillary software, I suppose), the Gendarmerie has realized huge cost savings from the beginning of their migration.
Canonical discusses this example in their case study, French National Police Force Saves €2 Million a Year With Ubuntu.
After switching from Microsoft Office to OpenOffice.org and Internet Explorer to Firefox, the police force decided to upgrade 85,000 PCs to Ubuntu Desktop Edition, removing its reliance on the Microsoft operating system almost completely. As well as simplifying maintenance and improving ease of use, Ubuntu Desktop Edition is saving the police force €2 million a year in licence fees alone. By repurposing 4,500 machines to act as local servers, it has also dramatically reduced its hardware expenditure.