Green’s Dictionary of Slang

1. a gold coin.

1663

A. Cowley Cutter of Coleman-street (1721) 763: I’ll [...] presently go put five hundred marigolds in a purse for you. [Ibid.] 787: Here here the five hundred Marigolds.

2. £1 million.

1859		Hotten Dict. of Modern Sl. etc. 62: MARYGOLD, one million sterling.
1864, 1867, 1870		Hotten Sl. Dict. [as cit. 1859].
1873		Sl. Dict.
1919		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’.