Well, according to this [1] the Mac install base is around 66 million users. So, if 20% of them bought your app for $1 you'd be $13 million dollars richer. That seems worth it to me. It seems that you are underestimating the size of the market and potential the to make money by writing applications for the Mac. You are also ignoring the fact that Macs sell at a premium so those 66 million represent not the bottom end of the market, but those with money to purchase services to make their lives easier.
My estimate of 75 million was conservative, it’s closer to 80 million.
Half of all new Macs are sold to people who are ‘new to Mac’[1]. Even the people who buy a replacement for their old Mac will often find other uses for that old Mac or they will sell it or give it away.
One year a go, Mac install base was 66 million. Since then, 17 million Macs have been sold. 8.5 million of those are sold to people who never owned a Mac before. Let’s imagine that all the remaining 8.5 million units are bought by existing Mac users, and that half of them are bought because the old Macs died. That gives us a new total of 78.75 million.
[1] http://aaplinvestors.net/stats/mac-installed-base/