Learner's definition of COSTLY
[also more costly; most costly]
1 : having a high price : costing a lot : expensive
We use less costly materials in our products.
It was too costly to fix her car after the accident, so she decided to buy a new car instead.
2 : causing people to lose something or to suffer
They won the game, but their best player was injured, so it was a costly victory.
The decision to wait could be a costly mistake.