(individual) What does it cost you to sleep through one of 30 lectures in a course for which you paid $300 in tuition? What does it cost you to attend?
The cost is the knowledge one would have gained from the lecture, which could be worth a better grade, and better employment prospects, etc. To attend costs the student at least a good nap. Unless it's my EE240 lab teacher I had in 1995. In which case, you got a nap either way.