Pronunciation: (kôst'lē, kost'-),[key] —adj.,-li•er, -li•est. 1. costing much; expensive; high in price: a costly emerald bracelet; costly medical care. 2. resulting in great expense: The upkeep of such a large house is costly. 3. resulting in great detriment: It was a costly mistake because no one ever trusted him again. 4. of great value; very valuable; sumptuous.
5. lavish; extravagant.