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

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