Suppose Lamar decides to eat the vanilla custard. Which result would be a cost?

 - Lamar will give up the chance to eat the peanuts. The peanuts would have been healthier than the vanilla custard.
 - Lamar will get to eat the vanilla custard. Lamar thinks vanilla custard will taste better than peanuts would have.
This result is a cost. It involves giving up or spending something that Lamar wants or needs:
Lamar will give up the chance to eat the peanuts. The peanuts would have been healthier than the vanilla custard.