Simon is given a free ticket to see Coldplay Saturday night

Simon is given a free ticket to see Coldplay Saturday night. He already has a ticket to see Sting in concert that night. The Sting ticket cost Simon $50, although he would have paid as much as $80 to go to the show. Simon knows that he can easily sell the Sting ticket on Craigslist for $60. What is his opportunity cost of seeing Coldplay?

$20 (i.e. 80 - 60)