Answer this: "You won a free ticket to see an Eric Clapton concert (which has no resale value). Bob Dylan is performing on the same night and is your next-best alternative activity. Tickets to see Dylan cost $40. On any given day, you would be willing to pay up to $50 to see Dylan (because he's so cool!). Assume there are no other costs of seeing either performer. Based on this information, what is the opportunity cost of seeing Eric Clapton? A. $0 B. $10 C. $40 D. $50"
See if you can determine the correct answer before you look it up. There are links to more information in the form of pdfs, and a discussion in the comments. The problem seems to raise more questions that it answers. Link