Cost somebody dear
Definition 1 of 1
Pro Tip 1/3Emotional Weight
Emphasizes the emotional impact of the loss or harm someone has experienced. 
His trust in the wrong partner cost him dear, leaving him heartbroken.
Pro Tip 2/3Negative Outcome
Used to highlight a negative consequence, often avoidable, resulting from an action. 
Choosing to cheat on the test cost her dear when she was expelled.
Pro Tip 3/3Severe Consequences
Implies that the consequences faced are particularly severe and significant. 
His habitual lateness eventually cost him dear, losing him his job.