There may be other ways to package it before releasing it for free as well. Remember, patio11 manages to charge mostly teachers $30 (although not monthly) for bingo card software, but it took him 4 years of learning and optimizing to get there.
I think the difference is that he's providing something teachers already use, need, and look for, as opposed to a "new" kind of software solution. (Online gradebooks aren't that new, but they still are not in use by the majority of teachers.)
Maybe I didn't hit on this hard enough in the blog post—teachers just are not a ripe audience for software in general.