know/find out/learn etc something to your costSUFFERto realize something is true because you have had a very unpleasant experience 付出了代价[吃了苦头]后才知道某事 Driving fast in wet conditions is dangerous, as my brother discovered to his cost! 雨天开快车很危险,我弟弟吃了苦头之后才明白!
→ know/find out/learn etc something to your cost at cost1(2)