Well one way to lower it is to settle law around the death penalty it seems

Or you could just not kill people.

Using conservative rough projections, the Commission estimates the annual costs of the present system ($137 million per year), the present system after implementation of the reforms … ($232.7 million per year) … and a system which imposes a maximum penalty of lifetime incarceration instead of the death penalty ($11.5 million).

From amnesty USA. https://www.amnestyusa.org/issues/death-penalty/death-penalty-facts/death-penalty-cost/

Ted Kaczynski lived until 81 and absolutely deserved death.

And he did die. Does that not satisfy you?

Kidding, but it's not a matter of deserves. It's about the states power in relation to their citizens. The state shouldn't have the power over life and death, because power corrupts. Cases like this: https://innocenceproject.org/melissa-lucio-9-facts-innocent-woman-facing-execution/

The poor woman was interrogated for 5 hours straight by police into confessing her "crime", while pregnant with twins, after which she was sentenced to death (still alive btw, lawsuits still ongoing and sucking up taxpayer money, even 13 years later.). One of the influential things in her death was the District Attorney who was attempting to be reelected on a "tough on crime" platform.

Cameron County D.A. Armando Villalobos was running for re-election and seeking a “win,” and is now serving a 13-year federal prison sentence for bribery and extortion.

Of course, you made an argument about "what if we require really, really hard evidence"... but what evidence is greater than a confession? What if evidence is fudged? There can never be a guarantee, and we should design our systems to account for human error... or malice.

Prison should be a place to rehabilitate people first, and a place to remove dangerous people from society second. Not a political platform, like the death penalty is so often.

The death penalty is the ultimate form of virtue signaling. An expensive way to remove someone from society, when life in prison would have the same effects, relatively. Everybody dies eventually, no need to waste money on killing people early when we could be spending money on keeping people alive.