The U.S. Government Accountability Office has released Financial Market Value of Government Information Hinges on Materiality and Timing, a 34-page report on this practice, trying to figure out how pervasive the scam is. They didn't get any great answers:
"The political intelligence industry is flourishing, enriching itself and clients in the stock market, yet the report notes that it could not document who these people are or how much they profit," [Craig Holman, government affairs lobbyist for government watchdog Public Citizen] said. "Without full transparency of the activity of these political intelligence consultants and their clients, it is nearly impossible to know if they are trading on illegal insider information."