How? Is there evidence that backs this up?
Yes, there are decades of research backing this up.
Looking at the US:
- A legislative audit in Kansas found that the estimated cost of death penalty cases was 70% more than comparable non death penalty cases.
- In Tennessee death penalty trials cost an average of 48% than trials where the prosecutor sought life imprisonment.
- In Maryland death penalty cases cost three times more than non death penalty cases.
- As per a 2008 California study, the then system with a death penalty cost $137m per year, whereas the estimated cost of a non death penalty system was $11.5m per year.
And so on.
The cost of appeals is a big factor. But (according to Amnesty at least) most of the costs come pre and during trial, so even if all post-trial appeals were abolished it would still cost more. Reasons for this include the increase complexity of the trials (they essentially become two parts, one establishing guilt/innocence and the other whether the death penalty is warranted) with all the attached special motions and extra time expended, increased investigative costs on the part of all parties and then the costs of life imprisonment
anyway whenever verdicts less than death are ultimately reached.
And
then any post-trial stuff, which can be significant not least because a third of death penalty verdicts between 1973 and 2013 in the US were overturned, often following years and years of expensive proceedings, many of which end not with the release of the prisoner but with the continued cost of their life imprisonment.