I don't buy eBooks. I prefer real books if it's a subject that requires 250 or more pages. Part of this might be that I'm tired of staring at screens at work all day or that I want a physical asset for my money. Under 250, I think I can find all the information I need through internet sources like articles, forums, blogs, etc. I once used a free eBook to learn C#.
I'm not sure what constitutes a mini course. I have done free coursera courses. I also paid about $30 for a practice AWS test set on Whizlabs.
Especially satisfied with Udemy : I heard a lot of "everything is on youtube, google the rest" but having a structured course really helped me get into programming. For 10$ (wait for the weekly "sales", that's the real price) you have access to a great educator who can afford to invest more time into his lessons than a youtuber or in a medium post.
Unfortunately I can't seems to find valuable ebooks or courses for more advanced topics...
I have paid for like 3 classes, each under $20. Two I was happy with one I felt a bit cheated.
[1] https://www.udemy.com/course/ios-13-app-development-bootcamp...
That's about it though.