Sara has recently become unhappy in her relationship with George. It seems to her that she is putting all the effort and resources into the relationship. She works two jobs. George goes to school, but does not even have a part-time job. He goes out with his friends on the weekends, while she stays home cooking and cleaning. Sara is wondering if the benefits of being with George outweigh the costs. Which theory is Sara using to determine if she wants to stay in or leave her relationship with George?