$39 seems fair, though I'd rather see this as an open-source project that the community could contribute back to. According to the changelog, this is a v1 release and the website contains no statements about the $39 buying future updates for new features or updated versions of Sketch, which is disappointing.
Aside from a few screenshots, it would be nice to have a YouTube video demonstrating how it works in practice, in real-time, especially considering the terms stating: "As a customer you are responsible for understanding this upon purchasing any item from Sketch Design System".
I wouldn't buy it without more information, and knowing who is behind it. There's no "about" section on the website with a real person identified. The domain WHOIS data is a proxy service.
You're basically paying $39 for the file setup/template and expected to fill in the components or elements yourself. Essentially you have to do all the work anyway so you're better off saving the money and building your system on your own anyway. There isn't anything in the purchased file that is all that helpful unless you're an absolute product design noob with no understanding of how systems work (in which case you should do that research instead of purchasing anyway).
Can you shoot me an email? Link's in my profile. Would love to pick your brain about a few things.
Invision beat me to it: https://www.invisionapp.com/blog/design-systems/ and also this site [probably the most comprehensive IMHO] lists lots of resources: http://styleguides.io/examples.html
Takedown notice will follow...