If I got 1,000 visits per month to an ebook site through content marketing, I could only count on about 2% joining my email list, and around 9% of those ever buying (3% across three pitch emails). Multiplying those, content marketing would only drive 1.8 sales per month. Clearly some people are finding much more success than that - which of these numbers is off?
One thing to keep in mind is that 1,000 visits is not very much traffic. Your other numbers are probably off slightly, especially since you can upsell people who do buy in order to generate more revenue, but more importantly, in order to make money with content, you'll need to generate a lot more than 1,000 visitors per month.