Green’s Dictionary of Slang

cow n.2

[ety. unknown; ? related to pony n. (1b) etc]

£1000 sterling.

[UK]Athenaeum 10 Sept. n.p.: ‘Liverpool.’ All over Lancashire a horse is called a cow, which everywhere else where slang prevails is a cant term for a thousand pounds [F&H].
[Aus]Smith’s Wkly (Sydney) 7 June 9/6: Slang of Money [...] A million is a ‘marigold’; £100,000 is a ‘plum.’ £1000 is a ‘cow’.