Green’s Dictionary of Slang

grey mare n.

the fare.

1932		‘P.P.’ Rhy. Sl.
1935		Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 5: Grey mare: Train or bus fare.
1960		J. Franklyn Dict. of Rhy. Sl. 72/1: Grey mare Fare (tramcar or omnibus).