Nightmare On Wall Street: Prosecution Witness Describes 'Chaos' In UBS PaineWebber Attack - Yahoo! News
Article about a bank who suffered from an internal attack.
Regardless of whether the guy on trial is guilty or not, the numbers mentioned in this case are a good point for why internal security is something which requires a lot of attention. From the article, the defence case seems to revolve around the fact that the network had very poor security and as such there's no way to prove that it was the defendant that placed the malicious code...
the estimate is that $3.1 million was spent on repairing the damage alone and I'd guess that the loss from lost business and opportunity cost of people not being able to use systems is going to be pretty high as well.