From Skeletons on the Zahara: A True Story of Survival, by Dean King (Little, Brown, 2004), Kindle pp. 147-149:
As Riley was about to discover, a Christian’s value as a ransomable commodity depended on his rank, wealth, health, and location. On the desert, where a tent with a life of four years was worth a camel, and a camel was worth a dozen goats or half a dozen sheep, a Christian’s worth fell somewhere between a tattered blanket and an adult camel, except in rare circumstances. Officers were worth more than seamen, though the Arabs, desperate for practical skills, would hold indefinitely a gunpowder maker, a surgeon, or a smith who naively admitted it. Married men brought more than single men for their perceived added wealth. The Arabs quickly noticed a man’s fine accoutrements. Brisson, who had lavished watches, silver buckles, and money on his first captor to ingratiate himself, was sold from one owner to another for five camels, while the ship’s baker went for one. Ultimately, Brisson regretted the gifts, which served only to inflate his ransom price.
To ransom a Christian, a Sahrawi had to deliver him to the imperial port of Swearah, where foreign merchants or consuls could make the payment. To get there, they had to cross the desert, past hostile bedouin tribes, past the fortified Berber towns of the Souss region, and finally past the operatives of the Sultan of Morocco, where Christian slavery was technically illegal and the sultan was fond of the “gifts” Western nations paid for their citizens’ freedom. All the while, the captor had no guarantee he would actually receive the agreed-upon sum. Instead of making the long, risky journey, a Sahrawi often sold his slave locally at a small but sure profit to a buyer who would sell at a small profit to another buyer.
In this way, in an agonizing peristalsis, the Sahara slowly yielded Christians north one territory at a time, the nearer to Swearah the higher the price, with the medium of exchange switching from bartered goods to cash at Wednoon, on the edge of the desert. On the Sahara, the French merchant Saugnier was traded once for a barrel of meal and a nine-foot bar of iron, and later for two young camels. He was sold twice at Wednoon, first for $150, then for $180. Seamen with him brought $50 to $95. Robert Adams of the Charles went in the latter range, once for $50 worth of blankets and dates and a second time for $70 worth of blankets, dates, and gunpowder.
In 1810, the English merchant and author James Grey Jackson proposed paying a fixed rate for Westerners delivered to Mogadore. “A trifling sum would be sufficient,” he maintained, if it was always on hand and the policy well known. This would eliminate the uncertainty that led to the repeated reselling of Christians and extortionate ransom prices. Jackson estimated that $150 per man would be enough, “a sum rather above the price of a black slave.” The British adopted the practice to the south at Saint-Louis, on the Senegal River, where in 1816 the speedy recovery of some of the passengers of the Méduse proved its soundness, but no such standards existed for Christians being transported north.