Estimating the size of the illicit market is a difficult undertaking. Skene writes "Interpol ranks art theft as the fourth largest criminal enterprise after drugs, money laundering and weapons." This appears incorrect, but its a common mistake. A number of media reports and even scholarly articles use this ranking, but I'm not sure its accurate. Interpol certainly does not endorse it:
We do not possess any figures which would enable us to claim that trafficking in cultural property is the third or fourth most common form of trafficking, although this is frequently mentioned at international conferences and in the media. In fact, it is very difficult to gain an exact idea of how many items of cultural property are stolen throughout the world and it is unlikely that there will ever be any accurate statistics. National statistics are often based on the circumstances of the theft (petty theft, theft by breaking and entering or armed robbery), rather than the type of object stolen.
The best estimates I have found are the FBI's rough account of $6 Billion annually, and the various reports given to the UK's Department of Culture Media and Sport Illicit Trade Advisory Panel which was given a number of very different estimates. I wonder, do any readers have any better or more concrete estimates? Empirical research is very popular in legal scholarships these days, does anyone have any ideas about how we could calculate the size?