big bucks vs megabucks

big bucks and megabucks both are nouns.

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

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

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

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

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