dear vs costly

dear is a noun but costly is not a noun.

Word	Noun	Adjective	Verb	Adverb
dear	Yes	Yes	No	Yes
costly	No	Yes	No	No

As adjectives, dear and costly are synonyms defined as:

Other synonyms of dear include high-priced, pricey, pricy.

dear (noun)	costly (noun)
a beloved person; used as terms of endearment
a sweet innocent mild-mannered person (especially a child)

dear (adjective)	costly (adjective)
with or in a close or intimate relationship	having a high price
having a high price	entailing great loss or sacrifice
dearly loved
sincerely earnest

dear (adverb)	costly (adverb)
at a great cost
with affection