[Brutha]

To give you some numbers, most offline services are able to sell their paid plan to around one percent of the people who get the free plan.
That should be your goal.

But I don't think you will get 300$ for a yearly subscription for content you produce yourself parttime.

Making money from Amazon/Google is probably the better plan.

Maybe you can find a way that people can only take your course when they buy the course book through your amazonshop?

If 100 people buy three books a year for your courses a 10$ you gain 20% from amazon. That means 100*0.2*3*10$=600$.

With a paid plan you get only on person for the same courses that pays you 300$.

I would rather try to make the money through the Amazon route.