megabucks vs big bucks

megabucks and big bucks both are nouns.

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

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

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

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

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