The Business of Botnets
It can be lucrative:
Avanesov allegedly rented and sold part of his botnet, a common business model for those who run the networks. Other cybercriminals can rent the hacked machines for a specific time for their own purposes, such as sending a spam run or mining the PCs for personal details and files, among other nefarious actions.
Dutch prosecutors believe that Avanesov made up to €100,000 ($139,000) a month from renting and selling his botnet just for spam, said Wim De Bruin, spokesman for the Public Prosecution Service in Rotterdam. Avanesov was able to sell parts of the botnet off “because it was very easy for him to extend the botnet again,” by infecting more PCs, he said.
EDITED TO ADD (11/11): Paper on the market price of bots.