As a nitpick (it only scales the numbers a bit) I would guess the proportion of paying readers is actually quite a bit higher than 1:650. The "650,000 readers"

As a nitpick (it only scales the numbers a bit) I would guess the proportion of paying readers is actually quite a bit higher than 1:650. The "650,000 readers" figure quoted here appears to be total hits to the website, which is an extremely optimistic count of the number of readers. It's almost certain that a large proportion of those did not read the book at all, and therefore it's unsurprising they didn't pay for it, either. Raw hit counts include people who just open every link on an HN or Reddit page in a new tab, and may or may not read the resulting tab; it also includes people who clicked on a Google search result and immediately bounced when they realized it wasn't what they were looking for. It's hard to get good numbers on this sort of thing, but one proxy that's sometimes useful is the number of visitors who clicked on anything beyond the first page they landed on.