The kicker here is that word "might". I'd willingly pay $39 for a book that will get me thousands of extra dollars; I wouldn't for one that will do so with probability 0.1%. The truth is probably somewhere in the middle; perhaps the book is worth every cent it costs and much more; but it's no good just to say "it might make you thousands" as if that settles the matter.
Actually, by your logic, it is. If the book stands to possibly make you $10,000 with probability .1%, the book is worth $10. As you mention, the truth is probably somewhere in the middle. I'd conjecture the book is worth at least $39 if it stands to make you $10,000+ with greater than .1% probability.
Also, if the cost of a $39 book that might be able to help you raise your rates, thus resulting in thousands of extra dollars is too high... Well :-)