1. most expensive; requiring the greatest amount of money
Usage: superlative form of costly
2. causing the greatest loss, damage, or sacrifice
Usage: superlative form of costly

Examples

• The diamond necklace was the costliest item in the jewelry store.
• This was the costliest mistake in the company’s history.
• The hurricane proved to be the costliest natural disaster of the decade.
• Among all the renovation options, replacing the roof was the costliest.
• The war became the costliest conflict in terms of human lives.
• She chose the costliest university despite having cheaper alternatives.