Portuguese insurance company Tranquilidade migrates desktops to GNU/Linux, reported a costs cut of 80%.
Here's a link to a presentation about it, there is a study case paper about it published somewhere, but probably in Portuguese.
https://www.slideshare.net/slideshow/desktop-linux-na-tranquilidade-opentech-2014/36740025
Maybe @gvlx knows where the paper can be found (or other migration stories with figures).