big money vs megabucks

big money and megabucks both are nouns.

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

As nouns, big money and megabucks are synonyms defined as:

big money and megabucks: a large sum of money (especially as pay or profit)

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

big money (noun)	megabucks (noun)
a large sum of money (especially as pay or profit)	a large sum of money (especially as pay or profit)