It seems safe to say that very few people on either side of this debate are chiefly interested in cutting costs. Rather, they are making a fiscal appeal in an attempt to win over those unconvinced by their moral one. For the anti-death-penalty side, however, the cost-cutting argument runs counter to their stated principles, at least if they claim to be promoting a more broadly civil-libertarian approach to criminal justice.
The concluding sentence of the Times article illustrates the contradiction, although the reporter does not seem to have noticed it: "In December  a state commission on capital punishment recommended that Maryland abolish the death penalty because of the high cost and the danger of executing an innocent person." But the cause of the high cost is precisely because the extraordinary efforts the justice system, state and federal, makes to ensure that innocent people are not executed.
If there are innocent people on death row, abolishing the death penalty would make them less likely to go free. They would be thrown back into the general prison population and deprived of the priority their appeals now enjoy because they are in danger of being executed. Their sentences presumably would be reduced to life in prison, a punishment that takes longer to carry out but whose end result is the same as that of execution.
Death-penalty opponents could argue that all convicted criminals deserve the same procedural protections that death row inmates now get. If that is what they believe, however, they are making their cost-cutting arguments in bad faith. If you care about justice for the wrongly accused, the costs attendant to the death penalty are money well spent.
Glad to see these points finally getting some major media attention. To the extent we spend more on capital cases to be sure we have the right guy, there is no moral justification for spending a penny less if the penalty is life without parole. An actually innocent lifer may have more time to prove his innocence, but with no resources to do so, the chances he will be able to are infinitesimal. Out of those people on the DPIC's "innocence list" who actually are innocent (which is not all or even most), it is safe to say that many would be in prison to this day if they had been sentenced to life instead of death.
But Taranto is mistaken when he implies that most of the extra costs are for certainty of guilt. In fact, the largest single item of higher cost is the Supreme Court's mandate that the defendant must be able to proffer anything at all as mitigation. This requires examination of the defendant's entire life by trial counsel preparing for trial and by habeas counsel making the knee-jerk attack on trial counsel's competence. The latter claim is typically examined by at least four different courts. All this litigation is a colossal waste of resources. Make the penalty depend solely on the crime itself and the defendant's criminal record or lack of one, and a big chunk of the expense will vanish.