A pretty penny
Definition 1 of 1
Pro Tip 1/3Idiomatic Expression
Understand 'a pretty penny' means a lot of money but is not meant to be taken literally. 
His collection of rare stamps must have cost a pretty penny.
Pro Tip 2/3Figurative Speech
Recognize it as an exaggeration; it suggests something is expensive but not necessarily pretty or a penny. 
That antique vase looks old but is worth a pretty penny.
Pro Tip 3/3Context Matters
Use it in informal conversations about expensive purchases or valuable items. 
I'd love to buy that painting, but it's a pretty penny.