I agree that it has been a very cost effective form of advertising in the past, but it's not free. Setting up and maintaining a botnet is likely to take a lot of man hours. You have to assume that there are people out there trying to catch you, so regularly have to shuffle your systems around to cover your tracks, At a guess one success in a million isn't enough to cover this - maybe it's one in ten thousand or less. And remember, nowadays with anti-spam improvements maybe less than 5% of the spam ever gets to the point where there's a chance that someone 'might' click on a link.
I can see how well crafted phishing attacks might make money, but I'm still hoping that perhaps broadcast spam just isn't worth it any more.