It is my understanding that the extreme expenses related to the death penalty are mainly due to the additional safeguards that are taken by the state to ensure that no innocent person is ever sentenced to death. You can, for example, check
http://www.deathpenaltyinfo.org/arti...08#From%20DPIC for more information. Although googling under 'death penalty cost' also reveals a wealth of information.
Some information copied from site posted above:
* At the trial level, death penalty cases are estimated to generate roughly $470,000 in additional costs to the prosecution and defense over the cost of trying the same case as an aggravated murder without the death penalty and costs of $47,000 to $70,000 for court personnel.
* On direct appeal, the cost of appellate defense averages $100,000 more in death penalty cases, than in non-death penalty murder cases.
* Personal restraint petitions filed in death penalty cases on average cost an additional $137,000 in public defense costs.