I thought the same - the list is not self-explanatory, nor does it argue that it is “cheaper”.

But if you click the link it will take you to a page that explains a bit more:

Overview

The death penalty is a moral issue for some and a policy issue for others. However, it is also a government program with related costs and possible benefits. Many people assume that the state saves money by employing the death penalty since an executed person no longer requires confinement, health care, and related expenses. But in the modern application of capital punishment, that assumption has been proven wrong.

The death penalty is far more expensive than a system utilizing life-without-parole sentences as an alternative punishment. Some of the reasons for the high cost of the death penalty are the longer trials and appeals required when a person’s life is on the line, the need for more lawyers and experts on both sides of the case, and the relative rarity of executions. Most cases in which the death penalty is sought do not end up with the death penalty being imposed. And once a death sentence is imposed, the most likely outcome of the case is that the conviction or death sentence will be overturned in the courts. Most defendants who are sentenced to death essentially end up spending life in prison, but at a highly inflated cost because the death penalty was involved in the process.

The Issue

How much the death penalty actually costs and how that compares to a system in which a life sentence is the maximum punishment can only be determined by sophisticated studies, usually at the state level. Many such studies have been conducted and their conclusions are consistent: the death penalty imposes a net cost on the taxpayers compared to life without parole. The question is whether the assumed benefits of the death penalty are worth its costs and whether other systems might provide similar benefits at less cost. The assessments of law enforcement experts are particularly relevant in identifying what expenditures are most effective in reducing crime.

That page unfortunately does not use references, so they don’t make it super easy to jump to the evidence that they base each statement on. But you can find a few specific cost studies here: https://deathpenaltyinfo.org/policy-issues/costs/summary-of-states-death-penalty, and I’m sure there are many more resources through the site if you are really interested.