Suppose you purchased a non-refundable plane ticket but, at the last minute, you have to cancel. You have the option to sell the ticket. If you initially paid $700 for the ticket, overnighting the ticket to someone through the mail would be $20, and you pay someone $10 to take the ticket to the post office to be mailed. What is the minimum price you should accept to purchase your ticket? (Hint: think about sunk cost)