It isn’t just the cost of incarceration. The cost of seeking a life sentence without possibility of parole, at least in my state, is substantially higher than merely seeking a life sentence. The legal hurdles are harder, the attorney time is longer, the standards are more stringent. In the vast majority of cases, the prosecution will opt for a simple prosecution seeking a life sentence, because seeking a life-without-parole sentence isn’t worth the additional cost.
And the cost of prosecuting a murder as a capital case is even more costly. It is seldom sought except in the most egregious cases. The most recent case I recall in this state is of the Carr brothers who were convicted of killing four people in cold blood after spending the night sexually abusing them. The state did end up with a conviction and a sentence of death, but I think it was a mistake to do so. They will be in prison at least twenty years before the appeals are exhausted, and probably longer. By then, the death penalty may be repealed.
The BTK killer who killed many over a series of years, did not get the death penalty, nor was it sought, because the crimes were committed at a time when there was not a death penalt in effect. He got a life sentence, and even with the possibility of parole, he will likely die of old age before the first parole hearing comes up.
I have no problem with the Church’s position on the death penalty, except this: To merely state that one can always incarcerate someone without parole is a lot easier to say than it is to attempt in practice. In practice it is expensive, litigious, time consuming, and more difficult than simply seeking a plain vanilla life sentence.