Interesting paper over at the Journal of Economic perspectives titled The Economics of Spam. The authors Rao and Reiley happen to be in research, the former working for Microsoft and the latter working for Google... not that they were collaborating from these two industry enemies - they both happened to be at Yahoo research when they wrote the paper.
What is interesting is that they find the cost to consumers and business of spam to be around $20 billion annually; while, the revenue to spammers in to be around $200 billion annually. Not a bad return given the burden they are placing on the world.
The rest of the paper is fascinating from a technical and economic perspective - check it out - The Economics of Spam.