An actual book written to be sold at a dollar isn't expected to be much. In fact, I would assume any book written for that price point is probably one of those

An actual book written to be sold at a dollar isn't expected to be much. In fact, I would assume any book written for that price point is probably one of those scammy "Send me $20 for a PDF all about being a video game wizard!" website type of books. However, for content that would otherwise be languishing, it can be a terrific move. Like Jerry Pournelle said, 70% is a hell of a lot more than you ever get from a publisher. Hell, 30% is a hell of a lot more than you ever get. Ten percent (what you'd get from a publisher) on a $20 book is going to get an author a couple bucks. On Kindle, they could sell it for $3 and still make more money. And at a buck or three, you're more likely to pick up more spontaneous sales and they can add up to ten or even a hundred thousand sales, pretty fast. If you are smart enough to have included a rights-reversion in your contract, it's a good way to make some additional income on what might otherwise be forgotten and ignored.