Green’s Dictionary of Slang

£1000, $1000.

1861		Bell’s Life in Victoria (Melbourne) 17 Aug. 3/3: Lord Dudley, who seldom has more than one bet on the Derby, and never less than a ‘monkey,’ offered to take three ‘gorillas’ about Dundee, but was not accommodated in a lump, though he could easily have obtained it ‘in driblets’.
1996		Ozwords Oct. 🌐 A monkey is $500 (formerly £500) and a gorilla $1000 (formerly £1000).
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 80/2: gorilla n. $1,000.
2013		T. Peacock More You Bet 67: ‘$1,000’ is a ‘gorilla’, or just plain ‘large’, as in ‘10 large’ for ‘$10,000’.
2024		J. Meades Empty Wigs (t/s) 247: He was attempting to raise funds [...] Had he not been such a boor I’d have stumped up what those coarse Cockneys call ‘a grilla’.