This page says he sold over three million books as of 2013: https://thewertzone.blogspot.com/2013/08/the-new-and-improve...

If he gets a dollar or two per copy (which I think is roughly correct, based on my very limited understanding of the industry) then that’s $3-6 million. He had been writing for around 30 years at that point, so that’s in the neighborhood of $100,000-200,000/year.