I think OS X sells for a very reasonable price. It is $149CAD, compared to $299 for Vista Ultimate (Upgrade).
Although of course, with Vista that is the last sum you will pay for the OS for the next five or six years.
£70 every 2-3 years isn't that much... It's less than 1/2 cigarette a day or 10p a day for 2 years.
I don't smoke. And I don't know what you are smoking, but OS X isn't updated every 2-3 years. It has so far been an average of sixteen months between updates from 10.2 to 10.4.