I think the moderators here have a set of relevant community links they can share, but to get you started with some official sources:
Section 5 of the instructor terms lays out the distribution of revenue:
https://www.udemy.com/terms/instructor/
It doesn't detail all of the "third party" fees that can be deducted, though. Typically these include app store fees and local taxes, and they can be substantial. The effects of local pricing and currency conversion can also be large. See also https://support.udemy.com/hc/en-us/articles/229605008-Instructor-Revenue-Share
The reason we give Udemy such a big cut is because they do a lot of marketing for us:
https://teach.udemy.com/teaching-on-udemy/how-udemy-markets-for-you/
It can't hurt to do some marketing of your own as well. Here are some resources on doing it yourself:
https://teach.udemy.com/marketing/
However, my personal advice is that marketing matters much less than your choice of topic to teach. THE MOST IMPORTANT THING is to research your topic in Udemy's Marketplace Insights tool:
https://www.udemy.com/instructor/marketplace-insights/
If you choose to teach a topic that is already saturated with established, well-reviewed courses, it's going to extremely hard for your new course to gain traction no matter how much external marketing you do. The trick is to find the rare topic that is rising in demand but underserved on the platform, and that you are qualified to teach.
Hope this helps.
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