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.
Keep in mind Udemy is already doing this for many courses. That is a big part of their business model, to do lots of advertising. So you might want to look at what kind of ads they are already running for similar courses (there are some tools that can do that, like spyfu etc)
Here's a guest post I wrote a while back talking about Udemy and how it fits into an overarching marketing strategy (like for example how you can have a free course that gets 10k students, promote your paid course to those students, and overall focusing on using udemy as a lead source to grow your main business)
That should give you some great ideas for where to start with this.
Had to share the google doc version because it looks like they migrated their blog when the company was acquired :)
(was originally here - https://wpcurve.com/online-courses-to-build-your-business/)
Hit me up if you want to talk course strategy / course marketing strategy - would be happy to help
Here's a case study that goes deep into course marketing with one of our clients (it's not about udemy, but should give you a clearer idea of the bigger picture with respect to how to market courses)