Sometimes the obvious stares you in the face.
One of the leading arguments used by death penalty opponents is that capital punishment costs too much.
Let's put to one side the fact that it costs so much mostly because those self-same opponents have spent decades larding it with manufactured procedural delays having nothing to do with either basic fairness or factual guilt.
Aren't the following two sentences a complete answer to the cost argument?
Expense is a reason to be selective and use the death penalty infrequently. It is no reason to make it legally unavailable, ever.

The Cost Argument
I am on an anti-DP mailing list: their motivation is moral.
Actually it ought be viewed as immoral, as it is a counterfeit
ethics, as Allah of the Koran is a warping of God of the Scriptures.
Since modern Western minds forget, pro- and anti-DP gits ought be made
to consider the following:
“THE murderer forfeits his right to life even in a state of nature,
where there is no civil authority …
In the yet uncultivated wilds of America, so strong are these moral notices, that the man who is conscious of having murdered another, does not pretend to resist the surviving relation … that universal practice which obtains among the savages,
of the nearest kinsman pursuing & slaying the murderer of his relation …
[due to] the law which God has written upon his heart.”
“These natural notions were planted in our breasts by the God who made us, and would for ever lead us to determine that the life of man is sacred, and his blood not to be shed, unless forfeited by some atrocious crime.”
Rev. J. Lathrop, 11 Mar 1770