"After deleting the 10,000th Viagra offer from your inbox, you might wonder, does anyone actually make money off this crap? Chris Kanich and his colleagues at UC San Diego and the International Computer Science Institute wondered too—so they hijacked a botnet to find out. Kanich’s team intentionally infected eight computers with a middleman virus, software they found in the wild that was relaying instructions between a botmaster computer and the network of computers it had secretly turned into spam-sending zombies. Then they changed the orders, effectively zombifying the botnet for their own research. Instead of sending hapless rubes to the botmaster’s website, spam ads would instead funnel them to a site built by Kanich’s team. It looked like an authentic Internet pharmacy, but instead of taking credit card numbers in return for a bottle of sugar pills (or worse), the site coughed up an error message and counted the clicks. Then the researchers calculated an estimate of how much money the spammer grossed per day: about $7,000."
Wired - Equation: How Much Money Do Spammers Rake In?