Researchers who infiltrated the Storm spam network have calculated that those sending unsolicited emails can make a profit by attracting just one sale from every 12.5 million spam emails.
Scientists from University of California, Berkeley and UC, San Diego (UCSD) hijacked part of the Storm network earlier this year for a month-long period, saying in a report (download PDF) outlining their findings that "the best way to measure spam is to be a spammer".
They used the network to send spam campaigns from a network of over 75,000 compromised machines. Despite sending almost 350 million email messages, only 28 sales resulted - a conversion rate of well under 0.00001 percent.
However, the researchers said that their study used only a small fraction of the overall Storm network. "We estimate roughly 1.5 percent based on the fraction of worker bots we proxy. Thus, the total daily revenue attributable to Storm's pharmacy campaign is likely closer to $7000 [per day]."
The report says that despite the tiny conversion rates, over the course of a year the Storm botnet could produce around $2.5m in revenue.
The researchers acknowledged that the figure was less than the "millions of dollars every day" that had previously been attributed to the Storm botnet network, but remained "a healthy enterprise".
See also: Storm Viagra spam generates £1.6m a year