Green’s Dictionary of Slang

(US) a deal, a bargain.

1869		‘Mark Twain’ Innocents Abroad 426: And each of the seven lifted up his voice and said, It is a whiz.
1876		(con. c.1840) ‘Mark Twain’ Tom Sawyer 272: ‘I’ll go back to the widder for a month [...] if you’ll let me b’long to the gang, Tom.’ ‘All right, Huck, it’s a whiz!’.
1911		S. Ford Torchy 147: ‘This’ll cost Cliffy just twenty.’ ‘I’ll pay it,’ says Mr. Robert. ‘It’s a whizz,’ says I.