Mark quit his job as a salesman where he made $43,000 per year to start his own t-shirt making business. His business expenses are $6,000 per year on rent, $12,000 per year on supplies, and $4,000 per year on part-time help. As for his personal expenses, his apartment costs him $4,800 per year and his personal bills are an extra $1,200 per year. What is Mark's opportunity cost of running the business?