Green’s Dictionary of Slang

1. ten shillings (50p) (cf. cow and calf n.).

1935		Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks n.p.: Cow’s calf: 10/- note.
1958		F. Norman in Sun. Graphic 20 July in Norman’s London (1969) 20: ‘Could you lend me a cows?’ asked some slag.
1962		F. Norman Guntz 8: [He] took a cows out of it and handed it to me.
1976		(ref. to 1930s–70s) R. Barnes Coronation Cups and Jam Jars 207: Cows – 10 shillings.

2. £150 [i.e. one and a half].

1989

in G. Tremlett Little Legs 193: cow’s calf £150 (i.e. one and a half).