That is why I didn't put it as dollars and cents. If Musk is teaching a class, whether to 100 or 10000 students the cost to him is his time. If he teaches for free then the students capture all the value. Or he could try to capture most of the value for himself. Think of it this way suppose there is total value (or utility) V created through teaching and the cost is C, then the value added is V-C. The value added could go to the professor or the students but either way there is a net benefit to lowering C.