If you say that something is costly, you mean that it costs a lot of money, often more than you would want to pay.
例句
Having professionally made curtains can be costly, so why not make your own?既然找专门人员做窗帘非常贵，为什么不自己做呢?
ADJ-GRADED
造成严重损失的;代价大的
If you describe someone's action or mistake as costly, you mean that it results in a serious disadvantage for them, for example the loss of a large amount of money or the loss of their reputation.
例句
Psychometric tests can save organizations from grim and costly mistakes...心理测试能够使组织机构避免犯下损失惨重的大错。This sort of scandal in international banking has been politically costly.国际银行业的这种丑闻一直以来都会造成极其恶劣的政治影响。