Your friend purchased a non-refundable ticket to a popular Broadway play for Friday night. Yesterday she was invited to spend the day on the lake with a group of sorority sisters. Her only cost will be for personal items such as lunch. The lake is two hours away from her apartment and the play is another hour in the opposite direction. She knows that if she goes to the lake, she will not get back in time to attend the play. Which of the following is not a relevant cost in making her decision on whether to go to the play or to the lake?