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The cost of the OS itself is insignificant.
You’re still paying for the hardware, still paying for warranties, still paying for support, and most importantly, still paying for IT staff. The cheapest platform is generally going to be the one that requires the least support staff to manage.
IBM has found that Macs are significantly cheaper in the long run, owing to increased productivity and reduced support costs. See https://www.computerworld.com/article/3452847/ibm-mac-users-are-happier-and-more-productive.html