Jon Brooks quit his job in a bicycle shop, where he earned $15,000 per year to become a graduate student in economics.

Jon Brooks quit his job in a bicycle shop, where he earned $15,000 per year to become a graduate student in economics. At the university he attended, he spent $2,000 on books, $1,000 on cough medicine, but he now earned $12,000 as a part-time economics instructor. What were Jon's opportunity costs of attending college?