Still, I found one website that cited various jurisdictions' data. The clearest citation I found was one from the Washington State Bar Association, which (when you add it all up) seemed to say that trying and upholding a capital case cost about $750,000 more than trying and upholding a non-capital case. That's an average.
Compare that to the Special Prosecutor's bill for the Plame investigation. The facts of the case were: there was no underlying crime to investigate in the first place; the SP knew who the actual leaker was that he was supposed to be finding (and the actual leaker was not charged with anything); the only guy convicted (and ruined -- the process is the punishment) was Scooter Libby, who wouldn't have committed any crime at all if he hadn't been rattled by the goons seeking what they already knew in pursuit of a crime that wasn't a crime. The bill for all that was $2.58 million.
We are brought up to believe that no price is too high to pay for justice. Still, there are some forms of "justice" we could do without. So, the next time somebody tries to say how expensive it is to prosecute capital cases, ask 'em how they feel about Special Prosecutors. If they think we've got the money to blow on stuff like the Plame investigation, then I find it hard to take them seriously when they gripe about how much it costs to punish murderers and terrorists.