I am curious too. Unfortunately, no way to find out the percentage number...
But sure enough - very large ROIs, the ''virtual'' nature of this business and in many times a grey zone jurisdiction, make this industry a very attractive target for such things.
They have backlinks from: washington post, telegraph, CBC, nature, healthline, the conversation, everyday health, bustle and so many others, the end price is more than fair, IMO it's pretty low.
If you think it's high then just check helptobuy.org.uk on Namebio. The value an authoritative backlink brings to a website is tremendous, there are businesses out there paying 5 figures for a SINGLE sponsored post.