You are not free to price your course anything you want. On the pricing page for your course you can select from a tier of prices such as $19.95, $24.95 up to $199.95 I think.
By default Udemy will market your course(s) for you but in exchange for that they will take 50% of every sale. If you make a sale as a result of an ad program Udemy is running, you will only keep 25%. If an affiliate of Udemy sells one of your courses, you will only keep 25%.
When Udemy is running a sale (which is probably every couple weeks) and Udemy is marketing your course(s) for you, they can choose to sell your course for anything they want, typically $9.95. But, this depends on the region the student making the purchase lives. In India, for example, the price is marked down even lower.
You can choose not to have Udemy market your courses for you but then you are responsible for your own marketing and most instructors are not well equipped to do that.
I would guess the average instructor who has Udemy market their courses for them earn about $4.00 per sale. This is what they actually keep. And it really doesn't matter what the list price on your course is because most of your sales will only come when Udemy is running some sort of sale. This means that a course that lists for $199.95 will be selling for $9.95.
This has been my experience and confirmed by others. Probably 90% of my sales come when Udemy is running a sale. And, when they do, my courses are marked down to $9.95, regardless of the list price I set. I have about 10 courses for sale and I've had almost 900 sales across those 10 courses. I average just about $4 per sale that I get to keep.