Let us remember that prison serves several purposes.
Punishment, and presumably some deterrent against re-offending and to others who might otherwise be inclined to commit a similar act is one aspect.
Rehabilitation, helping or making a person fit to live in decent society is another aspect. In many cases, time itself will work wonders on this. For a host of reasons, most 40 years are far less prone to various kinds of crime than they were when they were 20. And by 60 very few folks have much interest or ability to commit most violent or property crimes. (Pedophilia is one stark exception.)
The third and very important reason to put people into prison is to protect society until such time as they are no longer a danger. Maybe this is just a restatement of "rehabilitation." But in any case, it is distinct from punishment. In fact, in many cases I'm far less concerned about imposing punishment than I am in protecting society. Leniency for the battered wife who finally snaps and kills her husband in his sleep is, to my way of thinking, based less on any notion that "maybe he had it coming" than on "she probably doesn't pose much risk to others."
In any event, in capital cases we are rarely talking about a choice between rehabilitation and execution. If we (as society, the jury, judge, etc) thought there was half a chance at rehabilitation we'd probably not be looking at capital punishment anyway. Generally the question is between life without possibility or parole or other release and execution. I intend to avoid ever being faced with such a choice, but I'm thinking once the possibility of exoneration and release is gone (or effectively gone), that a quick and humane execution is probably no worse than and maybe much preferred over living live 23 hours a day in high security solitary confinement.
Finally, as we consider the cost of incarceration or any other punishment be sure to weigh it against the cost of not imposing punishment, not discouraging future crime, not protecting society from a convicted criminal. One death from a DUI car crash or any other violent crime can result in a couple of million dollars in lifetime lost income, or easily more than that in lifetime medical costs if the victim survives but is severely disabled. Not to mention intangible or difficult to compute costs like the increased risk of poverty or criminality among kids raised without both parents, loss of companionship, etc. A single small smash and grab at my home some years back cost me upwards of $500 plus the value of my time by the time I replaced a busted garage pedestrian door and several hand tools that were taken. My guess is that it supported someone's drug habit for all of about one day. Even if some "petty thief" only does something similar 2 or 3 times a week, you're looking at $65,000 in property damage and losses. Four times a week puts an annual total damages north of $100k. Plus the intangibles of the sense of violation. A few years later someone tried to kick in my front door, I presume to grab my flat panel TV, while we were out for an hour. My extra-long screws in the strike plate and hinges held. But the door and jam had to be replaced. Ever priced even a modest but half way secure front entry door? Failure to punish crime, to protect society from known, convicted criminals, costs a lot too. It just doesn't show up on government budgets quite so clearly.
Charles