First, I compared the Gatorade to a single lecture, not a single semester. Second, regardless of effort, there is a net value, and the net value of your average college course (what you learn less the effort and time required to learn it) is less than the net value of a Gatorade.

Finally, you can just change the thought experiment a bit. If you offered to pay someone twenty dollars to attend a random college lecture, would they? Probably some people would. And that fixes the upper bound of the value of the average lecture at less than twenty dollars, since, if it was that high, many people would be freeloading.