From A Labyrinth of Kingdoms: 10,000 Miles through Islamic Africa, by Steve Kemper (W. W. Norton, 2012), Kindle p. 110:
The unit of currency had changed to the cowrie shell. Cowries had reached Africa through Persia and the Maldives. Though heavy and inconvenient to lug around, the shells had been used as money for centuries throughout Central Africa, as far west as Timbuktu. They were also worn as ornamentation, like expensive jewelry. Small purchases—a needle, an onion, the small kebab that Barth enjoyed—cost a cowrie or two. Barth noted that a poor man could eat for five days on twenty-five or thirty shells, and a family could live for a year on 50,000 to 60,000, the equivalent of about £5. Larger items, such as a sword, cost 1,000 shells. A bull could run 7,000, a healthy young slave more than 30,000. In Tessaoua, Barth witnessed a major transaction in which half a dozen people did “the really heroic work of counting 500,000 shells.” He estimated that the current year’s rather scanty salt caravan from Aïr, consisting of 4,000 camel loads, would bring about 100 million cowries, or £8,000.