To your above question I would answer that I have advised past customers of mine who could afford it to buy a Mac, so they could escape the unreliability & security issues involved with the windows OS's. Everyone of them has been very happy with the change, & all of them are 3 years plus on their Macs, none use anti-virus software, none have had security issues, all are on their initial install & everything is going perfectly well on their machines last I heard.
Yes, those people paid more than they would have for a similar spec'd windows machine, but, they have not had ANY maintenance costs, or security issues. I think that, that, more than covers the cost difference for a windows to Mac conversion, most especially for people that don't maintain there own computers.
I know, the above did not answer the question you posed.
As far as whether it is worth buying a Mac as opposed to the PC/Linux combination is concerned? I would say that there are some ifs involved here:
If you are happy to use Linux? (Which is basically harder to learn & use than OSX & generally requires maintenance that OSX does not. This if, does have a few other nested ifs incorporated in it too.)
If you can do everything you need/desire to do on a Linux distro & the software available for it?
If the above ifs are ok with the user then I would say that generally a Mac is not the most economical solution.
If the above ifs are not ok with the user, I think that a Mac could very well be the most economical solution over the longer term.
For people that just want to use their computer & not be concerned about how it works, the Mac's with OSX really do make life very easy for them, imho.