Don't do it. Online ads have become less effective in recent years, and there's just no way you'll recover the cost of the ads for the few dollars you'll make from a sale on Udemy. Even Udemy has been shifting more toward the business-to-business world as they have been struggling to grow the marketplace, and their pockets are much deeper than yours.
Even for higher-priced items it is challenging. I recently ran a Facebook ad campaign for a $50 10-course bundle on my website, and even that couldn't break even. I even hired a social media advertising expert to fine tune the campaign, and it still lost money.
Stick with free channels, like links from YouTube video descriptions. Or just focus on driving organic traffic within Udemy by optimizing your course landing page and making your course as competitive as possible.
