From A Crime So Monstrous by E. Benjamin Skinner:
The market price for the guns was greater than the price for the children themselves. In 1986, an AK-47 cost ten cows, or about $1,000. By 2001, a glut of guns (and dearth of cows) shrank the cost to two cows, or about $86 along the Sudan-Uganda border. The child market was more volatile. In early 1987, after al-Mahdi began his counterinsurgency, a Dinka boy cost $90. By 1990, as supply swelled, the price fell to $15. At the time of my visit, CSI had agreed to pay $33 per slave.
Source: E Benjamin Skinner, A Crime So Monstrous: Face-to-Face with Modern-Day Slavery, (New York: Free Press,2008), page 82.