The very same question appears to have engaged the attention of computer scientists from the University of California, Berkeley and UC, San Diego.
They hijacked a working spam network and uncovered some of the economics of being a junk mailer.
What they discovered was that even a very low response rate netted these spammers vast amounts of money.
You can read more about the analysis here.
Image from Asylum Smiley
New Home Technology blog on the economics of spamming