Green’s Dictionary of Slang

1. costly, expensive, esp. over-expensive [the ‘salting’ of mines, thus the padding of bills].

c.1700		London-Bawd (1705) Ch. i: The Punk is her Salt Eel, and the Pander her Shark and her Swordfish.
1710		T. Ruddiman Gloss. to Douglas n.p.: Salt, I shall make it salt to you i.e. I shall make you pay dear for it [EDD].
1808		Jamieson Etym. Dict. Scot. Lang. (1879) IV 95: Salt [...] 2. Costly, expensive; applied to any article of sale.
1860		Hotten Dict. of Modern Sl. etc.
1866		Letters by an Odd Boy 163: Why should an extravagant bill be ‘rather too salt?’ .
1873		Sl. Dict.
1887		Fun 21 Sept. 126: A magistrate who was lately fined 20s. for striking a man in the street... remarked, ‘It’s rather salt’ [F&H].

2. (W.I. Rasta) unlucky, in a bad state; impoverished, empty-handed, low on food.

1976

A. Salkey Come Home, Malcolm Heartland 189: T’ings did salt, bad.