The word "opportunity" in "opportunity cost" is actually redundant. The cost of using something is already the value of the highest-valued alternative use. But as contract lawyers and airplane pilots know, redundancy can be a virtue. In this case, its virtue is to remind us that the cost of using a resource arises from the value of what it could be used for instead.
This simple concept has powerful implications. It implies, for example, that even when governments subsidize collegeeducation, most students still pay more than half of the cost. Take a student who annually pays $4,000 in tuition at a state college. Assume that the government subsidy to the college amounts to $8,000 per student. It looks as if the cost is $12,000 and the student pays less than half. But looks can be deceiving. The true cost is $12,000 plus the income the student forgoes by attending school rather than working. If the student could have earned $20,000 per year, then the true cost of the year's schooling is $12,000 plus $20,000, for a total of $32,000. Of this $32,000 total, the student pays $24,000 ($4,000 in tuition plus $20,000 in forgone earnings). In other words, even with a hefty state subsidy, the student pays 75 percent of the whole cost. This explains why college students at state universities, even though they may grouse when the state government raises tuitions by, say, 10 percent, do not desert college in droves. A 10 percent increase in a $4,000 tuition is only $400, which is less than a 2 percent increase in the student's overall cost (see human capital).
What about the cost of room and board while attending school? This is not a true cost of attending school at all because whether or not the student attends school, the student still has expenses for room and board.