1. Ask your audience; On IG I have put into stories, "would you prefer X or Y" and done so over the course of a few weeks to get some idea. Keep in mind you will be selling your course beyond whatever your current e-mail list or followers are.
2. If you haven't already, create a basic website and begin collecting e-mails (note: instructors can not collect Udemy students' e-mails), but you can collect from visitors and followers, I found this vital pre-launch.
3. I just did this with moderate success- send a quick survey and ask. I offered a $25 GC to one lucky winner. Sent 800 e-mails, got 18 replies.
4. A while back I also asked for Beta students on social media who would give more specific feedback, and rather than create four 6 hr. courses to gauge interest, I put one course's videos on DropBox and gave them a link and a google survey via e-mail. Once the course was live, all 10 Beta users received a free coupon to join and another (non-Udemy related) perk.
Rather than focusing on what your followers will pay, think about what Udemy users will pay (presuming your course will be public.) See what others in your field/topic are charging and go with that. Keep in mind most students will enroll during a sale (also presuming you are opting in for them), so you can price your course at $99.99, but the average student may pay $14.99 or less.
Alicia Paz M.A, QMHP