Given how slaves cost tens of thousands of dollars in today's money at the time of the American Civil War, I was surprised to read in a number of places (Wikipedia, Reuters, CNN) that the average price of a slave today is $90. The ultimate source for these claims appears to be University of Nottingham professor Kevin Bales. In a paper, he asserts that "slaves [are] less expensive today than at any time in the past", based on field observations.
Is this figure accurate? Are there independent sources that back it up?