megabucks vs big money

megabucks and big money both are nouns.

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

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

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

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

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