I've made over $8k since I launched last month and it's been a great stepping stone into other ideas. The main problem I see going forward is getting consistent traffic. Building the course was the "easy" part and now I'm focusing hard on SEO to keep consistent traffic going to my page. Long term I think I can make $100-$300 a day with it.
I floundered trying to build SaaS and other types of business for years before making some money online. Selling a one time purchase can be a game changer for those looking to go independent. Look at the stairstep approach[2] for reasons why this works so well.
Overall, if you invest a few days and put something out there, I think an online course can be a great way to build some income.
[1] https://www.vim.so [2] https://robwalling.com/2015/03/26/the-stairstep-approach-to-...