android is a bad example given that the money is made through advertising and selling your info.
a more reliable comparison is Apple and iOS. WP7 and the iPhone are limited use devices,
OSx and Windows on there respective hardware are computers with a significant use attached to them, the same can be said for Windows Tablets, however they are different from Apples ipad offerings which is just a bumped up iphone OS, the tablet is (at least we believe. x86 - yes, ARM - ?) its a fully functional computer
So to your question, a Windows Tablet, is a computer (probably), and thus will likely follow computer upgrade cycles. and ipad is an extension of its iphone and would follow the phone update model.
As to how much it should cost, well that's up to MS, but before folk start raving about Apples "cheap" updates, its worth pointing out that developing an OS with a fixed hardware design will be much easier (cheaper) to do than for an almost infinite number of hardware designs
Lets be clear on something, NOTHING is done without there being money to be made in one form or another, Googles model is not as transparent as Microsofts but it still makes a mint, and whilst it doesn't do that by charging you directly it does use you as its Bitch