I was thinking abour this earlier, and I think linux users feel "wealthier" in terms of software, and so are willing to pay more. Windows users have to scour the dark, virus-infected recesses of the internet to find free software. We just do apt-get.
I know of one person who paid $0.01 to download the Windows copies. (To be fair, he's in high school and doesn't have much money, but he could probably afford more than that.) I suspect there are more of these folks on Windows than any other OS.
I'd be curious to see the averages if the top and bottom 1% were omitted.
It would be interesting to see more complete statistics.