This answer will vary from person to person. 1. Going to the movies cost me two and a half hours of studying time but allowed me to have fun. 2. Sleeping for an additional hour allowed me to be more rested but prevented me from attending my friend's baseball game. 3. Attending school cost me several hours of free time but allowed me to do better in my classes.
Work Step by Step
Essentially, weighing marginal cost and benefit is like making a pros and cons list. In other words, doing one thing will prevent you from doing another. This is marginal cost. Doing that one thing can be a marginal benefit because you may have benefited in one way or another from doing it.