Suppose Tammy decides to borrow the book from the library. Which result would be a cost?

 - Tammy will give up the chance to keep the book as long as she wants.
 - Tammy will save some money by not buying the book.
This result is a cost. It involves giving up or spending something that Tammy wants or needs:
Tammy will give up the chance to keep the book as long as she wants.