Unlike reputable software development firms that develop and sell software, virus writers aren't concerned with ROI. They don't sell their viruses, so there are no concerns with cost of development, market share, pricing strategy, marketing, competitor advertising, etc. They write viruses to be malicious, to cause damage, to make a "statement", to feed their egos, and many other non-business related reasons. A virus writer doesn't get a return on investment in the traditional sense.
If anything, a virus writer would be highly motivated to have bragging rights to releasing the first virus in the wild that affects a large number of Mac OS X users. Such a feat would make them infamous overnight.