From Slash.org

"The NY Times has a nice feature about the diverging estimates of the costs of spam (Google). The estimates vary widely from $10 billion to $87 billion per year for American workers, and even more for global costs. Critics say that research firms' estimates vastly overstate the actual cost of spam. Public institutions like Indiana University have to be sensitive to the First Amendment rights of the spammers. And at companies like Nortel Networks, security architect Chris Lewis says that the real economic burden is the 10 to 15 percent - 5,000 to 10,000 messages a day - of the spam that still gets through, which costs the company about $1 in lost productivity per message. The costs can be much higher if a top executive is upset or mad about spam. "If someone in senior management gets spammed," Mr. Lewis said, "it could take 20 or 30 hours of everyone's time, up and down the chain." A chart of the per user amount of spam and the time spent processing it, as well as the varying estimates of the per user cost of spam are included in the article."