Couple things:
- There are different click rates and conversion rates for different products/services
- The biggest variable missing from your equation is the net revenue per sale. For example, if someone is selling an online course and makes a net revenue of $200 (lets' assume the price of the course is $279, minus $79 in costs to the seller (advertising ($38 or more), hosting, website, payment processing, etc.), then it's a great deal. Spend $38, make $200. Spend $38, make $200.
But you're right, you can't spend $38 to sell (for example) a $19 product/service. That math don't work.