Suppose Emmett decides to plant the irises. Which result would be a cost?

 - Emmett will give up the chance to look at the magnolia tree. He thinks it would have looked more beautiful than the irises.
 - He will save some space. The irises will use up less space than the magnolia tree would have used up.
This result is a cost. It involves giving up or spending something that Emmett wants or needs:
Emmett will give up the chance to look at the magnolia tree. He thinks it would have looked more beautiful than the irises.