I don't think he's in it for the money. I think that he, and many game developers of a similar vintage, are frustrated with the direction that modern software has taken. It's slow, getting slower, and often doesn't work correctly. I think this is his way of expressing that frustration, by teaching how the machine you're programming ACTUALLY WORKS. I for one have gotten more out of Handmade Hero than any other single source of learning in my life.
Maybe the ROI is the feedback and such from the community that has popped up around the game? That, and the $15.00 per copy to get access to the source code as the series goes on. But I'd be willing to bet that the feedback and community is the driving thing for this guy.