The least amount? My copy of OS X set me back nearly $2K when I got it over 2 years ago. Yes, it did include a computer that I can't upgrade (and won't shut down properly because of some Apple update early this year) and will be rendered into a large paperweight if either the computer or monitor kicks the bucket. Looking at the Apple Store site, the cheapest non-upgrade copy of OS X I can find is $599 with the typical price being at least $1199. Leat amount of money, my foot.
This is why Apple is okay only charging $29 for the upgrade; they already know that for 99% of SL purchasers, they already got a substantial amount of money. The fact that these people are continuing to throw money at them is just gravy.
You can't upgrade an all-in-one PC either. This is also one of the dumbest arguments I've heard since you imply the mac itself has no value. I love mac hardware.