knock back vs cost

knock back is not a noun while cost is a noun.

Word	Noun	Adjective	Verb	Adverb
knock back	No	No	Yes	No
cost	Yes	No	Yes	No

As verbs, cost is a hypernym of knock back; that is, cost is a word with a broader meaning than knock back:

Other hypernyms of knock back include be.

knock back (noun)	cost (noun)
	the property of having material worth (often indicated by the amount of money something would bring if sold)
	value measured by what must be given or done or undergone to obtain something
	the total spent for goods or services including money and time and labor

knock back (verb)	cost (verb)
cost a certain amount	be priced at
	require to lose, suffer, or sacrifice