What are the economics behind web spam? In this question, I'm referring to "web spam" on search engines (not email spam), such as placing misleading keywords on one's web page so that a pornographic site, for example, would get a lot of traffic that isn't necessarily related to what the site has to offer.

I am interested in understanding the dynamics, or rather, the economics behind this kind of activity, preferably with some numbers as examples or as data.

Links to studies are also appreciated.
I used to work for a major online advertising network, and it's pretty much an open secret in the industry that a substantial portion of ad impressions are either bot-generated, scams to exploit misleading search engine results, or both.  There are typically several layers of "middlemen" involved between the company whose products are being advertised and the websites running the ads, and everybody has a financial incentive to keep their mouths shut about the rampant fraud involved.

The basic economics is this:  an unscrupulous website joins an advertising network (which often have insufficient resources dedicated to their review/approval process for new websites joining their network) and then either purchases botnet traffic (at a lower price than they're receiving in ad revenue) or tries to pull in search engine traffic through misleading search terms, for as long as they can get away with it.  They're typically caught fairly quickly, but by that time they may have made several hundred (or several thousands) of dollars in profit.  Then they register a new scam website and repeat the process.  Remember:  websites are typically paid for the ads that appear on their website, even if nobody ever clicks on them.

Since nobody in the entire industry is really interested in exposing this aspect of the business, it may be difficult to find actual studies.  I'll take a look though, and would suggest that "fraud in online advertising" is the appropriate search phrase.

Update: It looks like the Wikipedia article on Ad Fraud has a number of references that may serve as a good starting point.  An online search turns up a good number of results as well, although many of them appear to be marketing material for software that claims to help prevent/stop ad fraud.