Jamieson, Fausset and Brown on 2 Kings 6:25

25. an ass's head was sold for fourscore pieces of silver--Though the ass was deemed unclean food, necessity might warrant their violation of a positive law when mothers, in their extremity, were found violating the law of nature. The head was the worst part of the animal. Eighty pieces of silver, equal to £5 5s.

the fourth part of a cab--A cab was the smallest dry measure. The proportion here stated was nearly half a pint for 12s. 6d.

dove's dung--is thought by Bochart to be a kind of pulse or pea, common in Judea, and still kept in the storehouses of Cairo and Damascus, and other places, for the use of it by pilgrim-caravans; by Linnæus, and other botanists, it is said to be the root or white bulb of the plant Ornithogalum umbellatum, Star of Beth-lehem. The sacred historian does not say that the articles here named were regularly sold at the rates described, but only that instances were known of such high prices being given.