Well, Windows is still the most popular operating system, which is over ten times more popular than Mac OS [1], [2], so if you want to have wider audience both for commercial and non-commercial targets, Windows is still your best friend.

However, I think there is an indirect factor, where I agree with Jason. Just think about it, who is most likely buy your commercial software, a guy that spent $500 on his Windows PC or a Mac "freak" who spent over $2000 on his Macbook Pro? Now, if you also consider people who replaced commercial Windows OS with free Linux distro, how likely are they going to buy it? This is an exaggerated speculation of course, but it's definitely an aspect that you need to analyze before releasing your software.

Also, there is another issue in mind - piracy. On Windows, it is incredibly easy to pirate, while, for instance, on an iOS powered device it may even result in warranty breach due to jailbreak.

Therefore, in my opinion, the "safest" commercial route for now is Mac OS and iOS, although for larger audiences, there is no beating Windows. Taking that in mind and back on topic, spending $500 on Windows-based PC, which can also sport Linux, can be considered money well-spent. Although, there are some Mac Mini's and MacBook Airs in $500 budget limit that have interesting hardware and should not be ignored!