How much money can a noob make on Udemy with a bellow average course?

It depends on the quality of your competitors, which niche you have put your course in, overall demand etc.

So you could have a below average course in a niche where yours is the ONLY course.

Then - even if the course is below average - if you offer print-your-own Diplomas were students can make up qualifications etc you could sell quite a few.

Maybe $500 a month….