Keep in mind that many of the people who would be most eligible for the death penalty probably would already need specialized holding facilities.
For example, Paul Bernardo (A Canadian serial killer who was involved in the deaths of several teenage girls, including his sister) is in isolation, since he might end up being a target if he were in the general population.
Is there a cost difference between a death penalty facility and keeping a person in isolation?
You know that's all the argument you ever need. And if opponents of the death penalty stopped there, I'd be happy. Its a convincing enough argument on its own.
If it is sufficient then the added arguments can not diminish it, even if they are only illusory by your counting.
Others don't see it as sufficient, some are willing to accept an imperfect system that metes out death to the innocent from time to time, but would be swayed by the financial considerations.
Each argument has an audience, and you may not be the audience for each argument.
If that's your argument you must be against putting anyone in jail, since arresting and incarcerating them is similar in practice to kidnapping.
There is a qualitative difference between killing some one and holding them against their will. If that escapes you, fall back on the death of innocents, you seem to grasp that one pretty well.
Keep in mind that many of the years a person will be serving a "life sentence" for will involve rather expensive medical care when the person becomes a senior.
A sad aspect of our corporate prison system is that they are actually very unlikely to make it to senior status and if they do we are very unlikely to spend much money on their medical care. Yes, you will find an odd example of someone living to an old age and the state spending exorbitant fees on their health, but I'd say it is not the norm and easily less than the total cost of defending the death penalty in court.
Remember, I've already brought up that issue... while I have read articles pointing out the costs, I figure that at least some of the higher costs are because capital cases are probably going to be complex based on the nature of the crime. (Another poster claims that the studies have taken that into account, which is quite possible, but I haven't seen it yet.)
A capital case triggers automatic appeals that are not in place if the prosecutor does not seek the death penalty. Those appeals alone trigger hundreds of thousands of dollars in expenses because the state pays both sides in many cases. As a tax payer it always pisses me off when I am paying two sets of attorneys to argue against each other, so there better be a very compelling reason to spend my money in that manner.
Can you provide that compelling reason?