costly vs dear

costly is not a noun while dear is a noun.

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

As adjectives, costly and dear are synonyms defined as:

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

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

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

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