[ewillbefull]

Interestingly botnets are probably a month away from being unprofitable for mining Bitcoins, in fact GPUs will be unprofitable for mining Bitcoins all-together. ASIC devices are being shipped over the next six months which will increase the mining difficulty substantially.

[LaRakel]

The benefit is that the electricity bill goes to the owners of the infected computers.

What does a botnet cost?

[eric_bullington]

I think the comment is referring to opportunity cost, since botnet's can also be used for other profitable activities, which may bring in more money than bitcoins (such as renting them to interested parties).

[avar]

Generating Bitcoins is CPU or GPU bound. There's a lot of profitable things you can do with botnets that don't compete for those resources.

[jbri]

There's opportunity cost in using it for yourself instead of selling the use of it to other people.