Many similar courses can be found for $10-$20 on Udemy.com (be sure to reset cookies/create a new account if you see higher prices. Apparently they only show discounted prices to new customer accounts.). For $600, a person could probably buy the majority or even entirety of Udemy.com's React courses -- Maybe somewhere around 500-1000 hours of content (rough estimation).
For example, this course has for 48 hours of content for $20. https://www.udemy.com/course/react-tutorial-and-projects-cou...
I've found Udemy has great content. Plus there's Youtube.
Kent, I understand that you're a solo entrepreneur and that you offer premium knowledge in your content. But I wouldn't pay more than $100. I don't think the knowledge you offer is monopolized by your course, I think it can be found through many other learning resources (books, videos, docs).
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Edit: just found out via another comment that Kent developed react-testing-library. That's awesome. So, Kent is likely someone who knows a ton about ReactJS, including truly expert, insider knowledge.
That said, $600 is still a crazy amount to charge to consumers. Maybe you'd charge that at an in-person seminar/workshop.
But to the internet public in general? Seems greedy.