Suppose Hansen decides to plant the oak tree. Which result would be a cost?

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