Please find a citiation for this study. Having a hard time believing the death penalty is more. If so, a bunch of people are ripping the taxpayers off again.
It's probably more, but nobody is ripping off anybody.
I always hate when I hear this "stat." While technically true, the reason that death penalty cases are more "expensive" is that plea bargaining - which happens is about 95% of all other cases and saves a ton of court costs - is non-existent since nobody will ever sign up to the death penalty. Also, since plea bargaining is out the window, a defense attorney is ethically bound to file any non-frivolous motion possible (a lot of cases have ripe legal issues but those are often never litigated because a plea bargain is reached). Trials will generally have way more witnesses and the bifurcation process includes a separate sentencing trial. Additionally, since a person's life is at stake and there is no downside to appealing, numerous appeals are often filed (at least one is mandatory). Accordingly, unlike in the average criminal case, a death penalty case is one where extensive litigation takes place throughout the entire criminal process. It's this part that becomes expensive since the Defendant has a right to counsel (e.g. in Phoenix a private attorney(s) is usually contracted to take the case and will usually do nothing but for a year and very justifiably bill about 150k), a prosecutor usually spends the bulk of his or her time for a year doing nothing but the case, experts are consulted and hired, and the court's time and man-power are substantially taken up.
Accordingly, while I don't doubt that it is more expensive on a per day basis to literally house an inmate, since that isn't an issue once a defendant is dead the only real "reason" why death penalty is more expensive of a punishment is because of the cost of the actual process since going through it all is inevitable. That said, if the death penalty were abolished and the max punishment is something like life without parole, the resulting costs would probably still be close to the same (although the costs of bifurcation and possibly the automatic appeal would be saved) since there is still no incentive to plead guilty. Trials would probably still be as long with full out litigation and then with a life sentence the state would incur the costs of incarcerating the guy until he naturally died - and then have to pay for his general healthcare as required by the 8th Amendment.
In my opinion there are numerous very good reasons for abolishing the death penalty. However, the "cost" of it versus lifetime incarceration isn't one of them and is a very misleading statistic.