Definition of costly adjective from the Oxford Advanced American Dictionary

     

    costly

     adjective
    adjective
    NAmE//ˈkɔstli//
     
    (costlier, costliest)
     
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  1. 1You can also use more costly and most costly. costing a lot of money, especially more than you want to pay synonym expensive Buying new furniture may prove too costly. Thesaurusexpensivecostly overpriced priceyThese words all describe something that costs a lot of money.expensive costing a lot of money; charging high prices:I can't afford it—it's just too expensive for me. an expensive restaurantcostly (somewhat formal) costing a lot of money, especially more than you want to pay:You want to avoid costly legal proceedings if you can.overpriced too expensive; costing more than it is worth:ridiculously overpriced designer clothespricey (informal) expensive:Condos downtown are now too pricey for local people to afford.Patterns expensive/costly/pricey for somebody/something expensive/costly to do something very/too/fairly/quite/pretty expensive/costly/pricey
  2. 2causing problems or the loss of something synonym expensive a costly mistake/failure Mining can be costly in terms of lives (= too many people can die).
 
noun [uncountable]
See the Oxford Advanced Learner's Dictionary entry: costly