This again? OSX versions are more incremental - part way between what MS would call a new OS, or a new Service Pack. MS don't charge anything for Service Packs.
Apple also have lower margins on their OS because it's built into the cost of their hardware.
Justify it any way you want to but MS is getting out competed in almost every area and that is just one more example. Apple gets a small sum every year form their customers and MS pisses theirs off with $140 price tags every 5 years.
Apple gets a small sum every year from those of us who bother to upgrade. But every few years they get much more than $140 out of us in the form of the purchase of a much higher-margin computer for which they don't have to split the profits with anybody else.
Also, let's compare apples to apples (ahaha). Apple only retails their OS in an upgrade so we can really only compare upgrade pricing. Apple charges about $20 for an upgrade every year, which averages out to $20/year. Windows 8 upgrades are selling for $40 right now, and the last version of Windows came out about 3 years ago. $40 / 3 = $13.33.
Now I absolutely agree with your assertion that $20 is a fairly small sum. But I'm not sure I agree that $13 is a price gouge by comparison.
Last night people were able to upgrade to Win 8 Pro for $14.99. You could enter ANY information and get the discount code. The deal seems to be dead now that they are asking for legit Win 7 keys. http://www.windowsupgradeoffer.com/en-US
Apple also have lower margins on their OS because it's built into the cost of their hardware.