A king’s ransom
Definition 1 of 1
Pro Tip 1/3Hyperbolic Expression
This phrase is often used to exaggerate the actual cost of something for effect. 
He spent a king's ransom on his wedding, or so he claims.
Pro Tip 2/3Literary Origin
Originates from historical ransoms for royalty; conveys a sense of extravagance and rarity. 
The necklace must be worth a king's ransom, it's so exquisite.
Pro Tip 3/3Informal Context
Used in casual conversation, not in formal writing or official financial discussions. 
My friend joked that his new phone cost him a king's ransom.