Whatever he earned, we know it's probably mostly gone by now. He has a (now ex) wife, two growing kids, and expensive habits to support. Book sales from something that is old news fast is nothing to rely on long-term, and especially for JWs... look at his second book, about "escaping" the JWs. That's now outdated with their changed rules on DFing and shunning. Even if he earned $100,000 from the book, that's a decent but not amazing annual salary for a family head here in the U.S. But even an amazing salary, you need to earn it every year, not just once and then you're set for life.
But like it's been brought out, he never thought the gravy train would stop rolling. He's like these actors or rappers or whoever, they hit it big with their debut or first role and assume they're gonna be millionaires for life, and then find out their follow up material is outdated or they get too big for their britches and on one wants to hire them, etc. Pretty soon they're declaring bankruptcy and having to sell off their ugly houses and gold teeth just to buy food.