I work for LANDesk, the premier desktop management tool in the world. There is much more to the cost of an operating system than, well, "the operating system". There are very few companies that have 100,000+ open source desktops. But there are many companies that have 100,000+ Windows desktops. Trust me, if it were cheaper to have an open source operating system, these companies would be the ones doing it.
How do you deploy the operating system?
How do you deploy software to the operating system?
How do you re-image the operating system when the user hoses it?
And when you re-image, how do you make sure that all the software that they should have is deployed to them with the new image?
How do many IT support calls do you take on the operating system?
How do you remote control these operating systems?
How do you manage security and patches on all these devices?
What is the cost to train an individual on these operating systems?
Windows is the most cost effective Operating System in all these areas.
It doesn't make sense to avoid the Operating System cost of $200-$300 for an OS license that you may even use for 10 years, but then spend $10,000 extra per PC per year to do all the tasks listed above. You will have to have more staff. Each FTE you add in IT is pretty much 100k after you factor in total cost of your employee (salary plus benefits, equipment, training, etc.). If it is a senior developer, the cost is closer to 150k.
Yes, all the above tasks can be done on Open Source platforms. But it isn't going to be as seamless as it is with Windows.
LANDesk's Management Suite helps IT departments manage desktops (and movile devices now) better than any other company, it is our job and our focus. We manage Open Source desktops better than most but all desktop management companies manage Windows more seamlessly.
And before you pass me off as just a Microsoft fan-boy, you should check out my blog, where you will see I am actually an Open Source fan-boy but I also have no hate for proprietary software like some do. I just love technology. The truth is truth no matter who you are a fan of.