Cost of customer acquisition will depend a lot on what type of course you're looking to sell and your target audience. No one will be able to give you a straight up answer here.
If you're able to provide more details, I could give some more nuanced advice :)
I would assume it depends on how crowded the topic your teaching is. If there are hundreds around the topic you would want to have a unique spin on it, even then it maybe hard to stand out, increasing your CPA (cost per acquisition) . Also udemdy takes a 90% or so cut from the leads they bring to your course.