money vs megabucks

money and megabucks both are nouns.

Word	Noun	Adjective	Verb	Adverb
money	Yes	No	No	No
megabucks	Yes	No	No	No

As nouns, megabucks is a hyponym of money; that is, megabucks is a word with a more specific, narrower meaning than money:

Other hyponyms of money include big bucks, big money, bundle, pile.

money (noun)	megabucks (noun)
the most common medium of exchange; functions as legal tender	a large sum of money (especially as pay or profit)
the official currency issued by a government or national bank
wealth reckoned in terms of money