Personal circumstances vary widely. The families of students on full scholarship face very different opportunity costs. Individuals with a trust fund provided by a grandparent may be able to draw down funds only for education. But for most of your families, college is a major expense. Your upkeep must be provided in any case.
Society makes the decision.
In addition, as above, you could be contributing your labor to society, so wages need to be added. Those wages should be gross of taxes and benefits, since you're generating the resources to provide for those even if you don't consume them directly. Hence staff salaries are not an opportunity cost, and we hould use only faculty salaries. Those resources likewise could be put to other uses. On average, the salary a college graduate receives is higher than that of a non-graduate.