Definition of costlily in English:

costlily

Pronunciation /ˈkɒs(t)lɪli/

adverb

rare
  • In a costly manner; expensively, lavishly.

    In later examples perhaps a typographical error for costily.

Origin

Late 15th century; earliest use found in Andrew of Wyntoun (c1350–c1422), prior of St Serf's, Lochleven, and historian. From costly + -ly.