Very true. I can confirm that Udemy is discounting courses all the time. There are time-limited offers for selected courses (that are regularly priced for 149$ to 499$) for 10$ oder 15$ at least once or twice a month. At least one teacher I know of has left Udemy for that practice, stating she feels kind of robbed because she can't deliver quality for a few bucks, but can't complete against all this bargaining offers effectively as well. Many teachers will offer participants of their other courses steep discounts as well, especially when starting new courses or updating older ones. The numbers of subscribed students seem heavily inflated for many courses by these practices: Just compare the number of (useful) reviews with the number of students subscribed.
I agree - the deep discounts lower average engagement by a lot. From what I've seen, most students who binge-purchase don't actually start learning from the courses they buy.