The short answer: both. Marketplace students want the best value for their money, and the Udemy Deals program ensures that most courses are "cheap" to begin with.
The longer answer: you need to study the existing top-selling courses in your category (they are easy to find via the Marketplace Insights tool.) If you expect your course to sell, it needs to compare favorably to whatever courses are already entrenched with large numbers of positive reviews, and large numbers of enrollments. That can tell you roughly how long a successful course needs to be in order to be competitive in your category.
I took a look at your course, and it's a fairly niche category that may not have much competition to begin with. If you're struggling with sales, I suspect it's not really about your price or your course length, but how many Udemy students are looking for what you have to teach in the first place. Again, the marketplace insights tool can help you understand the demand for your topic. Udemy isn't the right place for everything; if Udemy isn't the place people go to learn about mining, then Udemy might not be the place for your course.
Finally, if your course is destined for the Udemy Business program, then all bets are off. Udemy Business learners aren't paying out of pocket for their course, so they value their time more than the money. In Udemy Business, you may find that shorter courses attract more enrollments, because busy corporate learners can't afford to spend 20 hours taking some incredibly expansive course when all they need is some specific thing. However we are paid by the amount of time Udemy Business learners engage with our content, so you still don't want your course to be *too* short.
