Quite so, Kakato. Study after study have shown that death penalty is much more expensive that life without parole.
There are endless appeals in the case of death penalty, and attendant costs of lawyers, judges, other staff, cost of running the courts etc. Organizations like ACLU and others, who are strongly opposed to death penalty, have very smart lawyers. They keep finding loophole after loophole and keep launching appeal after appeal.
There is no such thing with life without parole. There is one appeal and that is it. So if one looks at cost alone, it is nonsense to have death penalty.
But influence of religion, of Bible is strong in USA. Bible says thou shalt have death penalty (At least that is how Americans interpret the Bible) and that is that. Common sense, cost effectiveness, these things do not matter when religious principles are at stake.