Word parts cost + -ly Pronunciation IPA (key): /ˈkɒst.li/ Audio (US)(file) Adjective Positive costly Comparative costlier Superlative costliest (formal) If something is costly, it costs a lot of money. Synonyms: expensive and pricey Antonyms: cheap and inexpensive Related words cost Wikiwand - on Seamless Wikipedia browsing. On steroids.