Analysts from from Ferris Research calculate that it costs US businesses some $8.9 billion annually to handle spam and European businesses $2.5 billion. They also estimate it costs ISPs an additional $500 million.
These costs are mainly in lost productivity in dealing with the mail, but they also include an estimated $3.7 in extra servers and other hardware to handle the spam load.
Marten Nelson of Ferris says that the spam problem is not as bad in Europe, hence the lower costs there. As wireless spam takes off in Europe, however, the calculations may need to consider these.