Just guessing based on rough numbers:
$400,000 / $20 = 20,000 purchasers at that $20 price point (which is probably low for the books I see around in the space). If you guess a conversion rate of 2-4%[0] you’re talking 500k - 1mm people. Obviously those numbers shift down with purchase price normalizing to what feels more standard in the tech space ($30-$40).
[0]: https://chrismcmullen.com/2017/11/03/book-marketing-by-the-n...