You didn't read up on how pricing and payments work. You are probably enrolled in Udemy's promotion program, which essentially gives them control of your pricing. On top of that, they can take up to 97% of your sale of a course, depending on how the student found your course.
Read through https://support.udemy.com/hc/en-us/articles/229232827-Instructor-Promotional-Agreements-and-Udemy-De... and https://support.udemy.com/hc/en-us/articles/229605008-Instructor-Revenue-Share to understand what is going on and what controls you have over it. And if you click on the $6 in your revenue report, it will tell you exactly how the money was divided up and the price the student paid.
But keep in mind that opting out of Udemy's promotions will make your course much less attractive to students; if they can buy an existing course that's on sale for $10 and yours is listed for $25, they're not going to buy your course. The way to make money on Udemy is through low prices and high volume. For example, I've amassed over 300,000 students on Udemy - even if I only receive a few dollars from each one, it adds up.
As the others have stated Louis, you won't get the full price for your course unless you opt out of Udemy's promotion programe. I believe if you opt out and if you drive the traffic directly to your course yourself then you get a higher percentage...97% *I think* (guess who else needs to swot up on how the payments work). If Udemy get the enrollment for you then they'll take half and courses are on offer to folks in different countries at different rates so you might make $5 from one enrollment in one country and then $3 from a different enrollment in another country.
I launched my first course at the start of this month and one sale gave me a whopping...$3.30!
I figure it's worth being a part of it though as Udemy can drive so much more traffic to my course (and any future ones I make) than I would be able to alone. And as Frank pointed out above, if you keep promoting it and engaging with people, it'll all add up.
Stick with it and good luck!