Apple upgrades OSX every 12 to 18 months and they cost $130 and nobody complain about that.
The other thing is that when you spend the $130 for the Mac upgrade, it's a complete system upgrade. You don't have to already have Mac 10.3 to install Mac 10.4. You can install the new operating system over the old one and keep all your settings, programs and files, or you can format the drive and do a completely clean install. It never checks to see if you have a previous version. Finally, to get a fully functional copy of Windows Vista, you have to shell out $400 (that's retail, and there will probably be some pricing discounts), but that's still a lot of dollars. For $200, you can buy a family license for OSX, and completely upgrade five Macs, with no requirements for previous versions, etc, for $200. That's half the retail price of one fully functional copy of Vista, and there aren't the myriad "Windows rules" in the Eula. Basically, you agree to not reverse engineer the OS or run it on more machines than the license allows. Other than that, if the OS is capable of it, you can do it. There's no "don't run the system in a virtual machine unless you pay for the full version and even if you pay for the full version, don't run DRM content on the virtual machine" caveats.