That's not really true: If you develop a nice and polished desktop application which can be better than an opesource alternative we are disposed to pay.
For example check the Humble Bundle: Linux users always pay more than average.
Although I'd not sell in the Ubuntu Software Center: As you would miss a lot of users from other distros (or people who don't use the Software Center).
Not quite true.
Linux users are the 'true average'.
The problem is, Windows Humble Bundles are bought for $0.01 en masse with the intention of reselling, sinking its average price.
For example check the Humble Bundle: Linux users always pay more than average. Although I'd not sell in the Ubuntu Software Center: As you would miss a lot of users from other distros (or people who don't use the Software Center).