If Sarah decides to spend her Saturday working on a freelance project that pays $150 instead of going to a concert that costs $50, what is her opportunity cost in terms of marginal cost?

Opportunity cost is what you give up when you choose one option over another. In Sarah's case, if she decides to work on the freelance project instead of going to the concert, she is giving up the enjoyment and experience of the concert, which costs $50. However, she is also gaining $150 from the freelance work. To find the opportunity cost, we look at what she is missing out on, which is the concert ticket price, but we also consider the money she earns. Thus, her opportunity cost is the enjoyment of the concert plus the $50 she would have spent, but since she earns $150, she actually gains $100 in total value by choosing to work instead.