I get why some people would not want to pay this, but it’s also not at all unreasonable to pay 50 USD.

It might be 3x a Netflix subscription, but Netflix is, for many, just wasting our time, whereas learning math could mean you can get a better job (higher salary, more interesting projects, future proofing yourself), then suddenly the 50 dollars per month is negligible.

I also get that in the end all this is available for free scattered around the internet and libraries, but having guidance, having a system that helps you actually do the learning is also very valuable.