Suppose Jen decides to plant the birch tree. Which result would be a cost?

 - Jen will get to look at the birch tree. She thinks it will look more beautiful than the sunflowers would have looked.
 - The birch tree will use up more space than the sunflowers would have used up.
This result is a cost. It involves giving up or spending something that Jen wants or needs:
The birch tree will use up more space than the sunflowers would have used up.