A tricky opportunity cost question

  I have recently listened an introductory section of the audio version of a book "The Economic Naturalist: Why Economics Explains Almost Everything". The author talked about "opportunity cost", which is a term we are all familiar with. The first time I heard this term was coming with an easy example. If a man wants to start a business, he should forfeit his current job. If the salary of his current job is $15000, then the opportunity cost to start a business will be $15000. Opportunity cost is the value one should give up in order to pursue an activity.

  I have not studied any economy courses in my school days but I still think the concept of opportunity cost is not difficult to comprehend. Then the author gave a far more tricky example. I was confused and finally took half an hour to contemplate. I say with my own wordings here but the concept behind is the same.

  You have just received a free concert ticket of singer John Lee. You cannot resell it. Before getting this free ticket, you originally wanted to buy a ticket of Mary Chan concert which will perform at the same night of John Lee concert. Mary Chan concert ticket is sold at $40 dollars. And in your mind, Mary Chan concert ticket is worth $50. The question is: What is the opportunity cost of going the John Lee concert? The options are: a $0, b $10, c $40, d $50

  This question is quite weird. Firstly, you do not know the ticket price of John Lee concert. Secondly, you have not bought the ticket of Mary Chan concert yet. Thirdly, the selling price of Mary Chan ticket is different from the price you give in your mind. The third point is quite funny for me. When we buy something, we consider not only the selling price, but also the price in our mind. I will buy a product which selling at $10 dollars and I think it is worth $15 dollars. If I think it is worth just $6, I will not buy despite $10 seems cheap. I will see the singer I love because a ticket selling at $100 is worth $200 for me. If I hate that singer, a $100 ticket is worth $0 for me and even negative(as I need to waste my time). Then, what is the opportunity cost of going the John Lee concert? Are the hints enough for you? Let me give the answer in the next post.