Definition of costly adjective from the Oxford Advanced Learner's Dictionary

     

    costly

     adjective
    adjective
    BrE BrE//ˈkɒstli//
     
    ; NAmE NAmE//ˈkɔːstli//
     
    (costlier, costliest)It is also very common to use more costly and most costly.
     
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  1. 1costing a lot of money, especially more than you want to pay synonym expensive Buying new furniture may prove too costly. Synonymsexpensivecostly overpriced pricey dearThese 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 (rather 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:Houses in the village are now too pricey for local people to afford.dear [not usually before noun] (British English) expensive:Everything’s so dear now, isn’t it? This word is starting to become rather old-fashioned.Patterns expensive/​costly/​overpriced/​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).
  3. Extra examples The six-month delay will be costly for the company. These measures could be costly to employers. These teaching methods are too costly in terms of staff resources. This process is costly in computer time. Privatizing the rail network turned out be a costly mistake for the government. The old palace may prove too costly for the Duke to maintain. You want to avoid costly legal proceedings if you can.
See the Oxford Advanced American Dictionary entry: costly