Final answer:
The difference between costly and expensive lies in their usage. Costly is used to describe something that requires a high price or high expenses, while expensive is used to describe something that has a high monetary value or is priced higher than average.
Step-by-step explanation:
Costly and expensive are both adjectives that describe something that requires a large amount of money to purchase. However, there is a subtle difference between the two. Costly is used to describe something that requires a high price or high expenses, while expensive is used to describe something that has a high monetary value or is priced higher than average.
For example, if a car costs $50,000, we can say it is expensive because it is priced higher than average. On the other hand, if repairing the car costs $10,000, we can say it is costly because it requires a high amount of money or expenses.