And all the work for the human rights lawyers (and rightly so, in this instance).
Google tells me this about the US: 'Death row litigation in the U.S. costs significantly more than regular life-imprisonment cases, averaging 2.5 to 5 times more per case and totaling billions of dollars nationwide over the decades. Key cost drivers include lengthy pre-trial preparations, specialized two-phase trials, and mandatory multi-layered appeals.'
If people are using the cost of imprisoning killers as a reason for the death penalty, it's clearly not that straightforward, if the US is anything to go by.