While I think banning the death penalty is the most preferable option, you could do it cheaply and humanely via asphyxiation. The feeling of asphyxiation is due to a build up of carbon dioxide, not a lack of oxygen. So, one method is to have the condemned breath helium or nitrogen. They'd feel euphoric, pass out, and die painlessly without feeling like they are asphyxiating because they'd still be able to expel carbon dioxide.
My only argument against the repealing of the death penalty is the exorbitant cost to house an inmate for life. Especially end of life medical treatments or cancer treatments. As far as punishment goes, life in a cage would be way worse than dying imho.
Life in prison is actually cheaper in the current system. Because of the permanency of the punishment, death row inmates are put through a rigorous appeal process as a matter of course.
But, to me, that's a red herring. As long as it is possible for the justice system to be wrong, I don't think the government should be granted the power to kill citizens.
I don't understand why quick mechanical humane executions aren't even considered. Or why they just don't give them an OD of a general anesthetic drug that just puts them to sleep and then stops their heart. It just seems like yet another money racket and bullshit red tape parade at present moment. And they don't even know if those drugs are truly painless or not (not that I give a fuck, but they make it a point that they are humane)