I was curious also so I ran some numbers:

He's the #1 search result for "personal development", which averages 90,500 searches per month from Google. Assuming he receives 60% of those visits, that's 54,300 visits per month. Since it's search traffic, which would be people willing to click and look for info, he might get a CTR of 2%, which is 1,086 clicks per month. The estimated CPC for that term is $1.92, so that is $2,085 per month, or $25,021 per year.

Since that's a rough estimate of $25k/year for one search term, I think $100,000 per year is reasonable.