The World Bank, UN Office on Drugs and Crime and Interpol recently published a book length study titled "Pirate Trails: Tracking the Illicit Financial Flows from Pirate Activities off the Horn of Africa." The study estimates that between April 2005 and December 2012, pirates claimed an estimated $339 million to $413 million in ransoms. This massive study looks at the role of criminal networks, distribution of the proceeds, moving the proceeds, investing the proceeds, and makes a series of recommendations for countering the issue of financial flows.
For journalistic accounts of the report, see "The Horn of Plenty: How Piracy Became a $413m Business" by Nick Kochan in the 4 November 2013 The Independent and "Somali Piracy: More Sophisticated Than You Thought" in the 2 November 2013 The Economist.