The California legislature found that spam cost United States organizations alone more than $10 billion in 2004, including lost productivity and the additional equipment, software, and manpower needed to combat the problem.
Spam’s direct effects include the consumption of computer and network resources, and the cost in human time and attention of dismissing unwanted messages. In addition, spam has costs stemming from the kinds of spam messages sent, from the ways spammers send them, and from the arms race between spammers and those who try to stop or control spam. In addition, there are the opportunity cost of those who forgo the use of spam-afflicted systems. There are the direct costs, as well as the indirect costs borne by the victims – both those related to the spamming itself, and to other crimes that usually accompany it, such as financial theft, identity theft, data and intellectual property theft, virus and other malware infection, child pornography, fraud, and deceptive marketing.
The methods of spammers are likewise costly. Because spamming contravenes the vast majority of ISPs’ acceptable-use policies, most spammers have for many years gone to some trouble to conceal the origins of their spam. E-mail, Usenet, and instant-message spam are often sent through insecure proxy servers belonging to unwilling third parties. Spammers frequently use false names, addresses, phone numbers, and other contact information to set up “disposable” accounts at various Internet service providers. In some cases, they have used falsified or stolen credit card numbers to pay for these accounts. This allows them to quickly move from one account to the next as each one is discovered and shut down by the host ISPs.
The costs of spam also include the collateral costs of the struggle between spammers and the administrators and users of the media threatened by spamming. 
Many users are bothered by spam because it impinges upon the amount of time they spend reading their e-mail. Many also find the content of spam frequently offensive, in that pornography is one of the most frequently advertised products. Spammers send their spam largely indiscriminately, so pornographic ads may show up in a work place e-mail inbox—or a child’s, the latter of which is illegal in many jurisdictions. Recently, there has been a noticeable increase in spam advertising websites that contain child pornography.
Some spammers argue that most of these costs could potentially be alleviated by having spammers reimburse ISPs and individuals for their material. There are two problems with this logic: first, the rate of reimbursement they could credibly budget is not nearly high enough to pay the direct costs; and second, the human cost (lost mail, lost time, and lost opportunities) is basically unrecoverable.
E-mail spam exemplifies a tragedy of the commons: spammers use resources (both physical and human), without bearing the entire cost of those resources. In fact, spammers commonly do not bear the cost at all. This raises the costs for everyone. In some ways spam is even a potential threat to the entire e-mail system, as operated in the past.
Since e-mail is so cheap to send, a tiny number of spammers can saturate the Internet with junk mail. Although only a tiny percentage of their targets are motivated to purchase their products (or fall victim to their scams), the low cost may provide a sufficient conversion rate to keep the spamming alive. Furthermore, even though spam appears not to be economically viable as a way for a reputable company to do business, it suffices for professional spammers to convince a tiny proportion of gullible advertisers that it is viable for those spammers to stay in business. Finally, new spammers go into business every day, and the low costs allow a single spammer to do a lot of harm before finally realizing that the business is not profitable.
Some companies and groups “rank” spammers; spammers who make the news are sometimes referred to by these rankings. The secretive nature of spamming operations makes it difficult to determine how proliferated an individual spammer is, thus making the spammer hard to track, block or avoid. Also, spammers may target different networks to different extents, depending on how successful they are at attacking the target. Thus considerable resources are employed to actually measure the amount of spam generated by a single person or group. For example, victims that use common antispam hardware, software or services provide opportunities for such tracking. Nevertheless, such rankings should be taken with a pinch of salt.
Source : wikipedia