I know I've spent more money specifically on GNU/Linux distros than specifically on Windows, buying various boxed sets and books-with-disks, but when an operating system is part of an OEM package, some costs are hard to tweeze out. (Not to mention whether, and how much, Windows users would have to pay for the functionality of the nice free, Free software that comes with typical Linux distros. And that in a workplace, support costs more than the OS's initial purchase price.)
jmcneal writes with this chin-stroker: "My colleages and I have been debating this for a while at work: 'Do people spend more money on Linux Distributions or on windows?' The limited sampling we have is that users buy distros almost every six months, at full price, at retail outlets. We have only one person who has gone out and purchased Windows at a software outlet, the rest of us only get a new copy when we purchase a new PC, about every 1.5 to 3 years. Is this behaivior common? How much have /.ers spent on distro's vs Windows in the last 2 years?"