Suppose Polly decides to buy a copy of the book. Which result would be a cost?
Polly will get to keep the book as long as she wants. Polly will spend money to buy the book.
Polly is deciding whether to buy a copy of a book or borrow it from the library. She would like to be able to keep the book as long as she wants, but buying the book would cost money. She could borrow the book for free, but she would have to return it in a few weeks.
Before you decide to do something, it is often helpful to list costs and benefits.
Costs are what you give up or spend when you decide to do something. Costs involve giving up things that you want or need.
Benefits are what you gain or save when you decide to do something. Benefits involve gaining something that you want or need.
This result is a cost. It involves giving up or spending something that Polly wants or needs:
Polly will spend money to buy the book.