A mac mini costs 600€ but it would be stupid anyway if there were platform dependent bugs. I canīt imagine that publishing to OSX needs a lot of testing.
Compared to Gamestudio there are a lot more possibilities to make money with Unity. iPhone games are very popular at the moment or you could do small web based advertising games. Itīs pretty easy to afford Pro after your first small indie project.
Assuming if..
1) Your game is good enough for that Mac mini.
2) Every Mac user out there has a Mac mini. If a more expensive (or "advanced") tech is common among users, wouldn't you rather have that to test your game onto? (why would you optimise it for lower end if that shouldn't be the target anyway)
3) If you know nothing about OSX. What happens when bugs are reported? You have to learn ins and outs of OSX now? If you hire someone for that.. well, he ain't working for free.
4) Lot more possibilities? What happens when your game isn't portable? Will you port an FPS to iPhone? What happens when your game isn't for browser based plugins? In the end you pretty much come down to downloadable standalone app.
5) It doesn't cost "600€" EVERYWHERE in the world (and 600 is still way too expensive, you can buy a pro license for that instead).
