From http://cryptome.org/isp-spy/yahoo-spy.pdf
Federal law (See 18 U.S.C. § 2706) requires law enforcement to reimburse providers like Yahoo! for costs incurred responding to subpoena requests, court orders, or search warrants.
• Basic subscriber records: approx. $20 for the first ID, $10 per ID thereafter
• Basic Group Information (including information about moderators): approx. $20 for a group with a single moderator
• Contents of subscriber accounts, including email: approx. $30-$40 per user
• Contents of Groups: approx. $40 – $80 per group
Yahoo is not the only company indulging in these activities. This is the reason having a personal profile on any kind of networking website like Facebook, Orkut or Linkedin is risky, you never know how it will be used or misused. Better to spend a little money and get your own website.