One computer at home.... Came with Windows (bought computer on ebay with genuine license). So I didn't have to buy separately. Work OS (2 computers) came with OS (I work in a Uni and they allow you to put XP on a home machine as part of their agreement (I don't, I have my own license that cam with the machine)) No cost to me. The MacOS machine did come with MacOS (of course) and it is a work machine so they bought the OS. No cost to me.
So the cost of the OS's to me are ZERO.
Surely you don't think someone that uses 5-6 computers actually bought the 6 from their own pocket?