This is a great post at Modern Life Is Rubbish. It is a really excellent summary of how Viagra spam works today and how the spammers make money. The reason it is such a great post is because it really sumarizes all the components of true Spam.
A quote from the posting:
Conversion rates are likely to be much higher - I would expect 1% to be a typical figure, but again, this can vary between campaigns. At this figure, then, the spammer can expect to see a handful of orders per million emails. Typical order values are quite high though - price breaks help push the lowest viable purchase to at least $100. If we assume $100 as the average transaction, 40% commission means the spammer will net $80 per million emails sent - all considered, not a bad return.
The post goes on to show the potential earnings by Viagra Spammers. The most interesting part of this post is how is shows that some of the most highly regulated and controlled businesses (email, ecommerce, pharmacological products, etc...) are being circumvented by some kid in his parents basement in Germany.