In addition to Kent's and Bill's cogent arguments about the cost of the death penalty, let me add another:  Justice is worth spending money on.   

We have Fourth, Fifth and Sixth Amendment rights not because they are efficient, expeditious or easy but because our forefathers recognized that justice isn't economical.  We could easily dispense with the cost of the criminal justice system and just civilly commitment people who are prone to violence and crime (abrogating Foucha v. Louisiana in the process, of course).   

But we don't because we understand that such a system would be unjust.  

Justice for society and victims is worth spending money on just as it is worth spending resources for the accused.