Symantec puts a number on the size of the market that trades in stolen credit card numbers and exploit code.

Measuring the Market for Hacked Data - BusinessWeek

Cyber criminals have assembled a black market for information worth $276 million, according to a new study by computer security company Symantec, which is spotlighting the results as these underground transactions drive more hacking attacks.

Symantec toted up the advertised prices hackers posted on Internet forums for stolen credit cards, bank accounts, and identification data; plus hacking tools and pirated software between July 1, 20007 and June 30, 2008. The company gathered the information by monitoring the publicly available chat boards used to make markets in stolen data and malware.
One phrase sticks out: "publicly available chat boards." Considering all the chatter we hear about private groups, how much do you want to bet that the number is actually much higher?