big money vs big bucks

big money and big bucks both are nouns.

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

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

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

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

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