Green’s Dictionary of Slang

(US) a large amount (unspecified) of money.

1957		‘Ed Lacy’ Lead With Your Left (1958) 91: She still thinks if you work hard you get ahead, make the big buck.
1969		‘Iceberg Slim’ Pimp 123: It had cost him a big buck to lower the ceiling.
1977		(con. 1960s) D. Goines Black Gangster (1991) 17: If you were going to live a life of crime, go for the big buck.