room rate vs charge per unit

room rate and charge per unit both are nouns.

Word	Noun	Adjective	Verb	Adverb
room rate	Yes	No	No	No
charge per unit	Yes	No	No	No

As nouns, charge per unit is a hypernym of room rate; that is, charge per unit is a word with a broader meaning than room rate:

Other hypernyms of room rate include rate.

room rate (noun)	charge per unit (noun)
the rate charged daily for a hotel room	amount of a charge or payment relative to some basis