Having dealt with the defense bar and its allies in advocacy groups and think tanks for over 40 years, I can attest that there is yet a third succinctly stated reason that the cost argument against the death penalty is tripe.
Any money the state saves by ending the maintenance of death row and the litigation entailed in capital cases will be sucked up -- every dime of it -- into the decades-long maintenance of killers serving LWOP, together with the litigation against LWOP as -- guess what! -- too expensive. That litigation will be undertaken by the many of the same people and groups who are now telling us, with their fingers crossed only half-hidden behind their backs, that the death penalty costs too much.
The coming tidal wave of anti-LWOP advocacy and law suits once the death penalty is repealed has already shown itself in too many articles to list. For a sample, read any three months' worth of Doug Berman's Sentencing Law and Policy.
Death penalty opponents are simply not telling the truth about what they actually plan to do with the LWOP they are now trumpeting as the alternative.