Academics at Berkeley used the "Storm botnet" network to blast 350 million emails for "male enhancement products" at a cost of about $80 per million emails sent. 28 sales resulted with an average purchase price of $100. They estimated that such campaigns when fully utilising the network could mean gross revenues of $7,000 to $9,500 a day, or $3.5 million a year for the spammers.
First read in The Register.
Full study here.