[hugh_]

It's largely economics of scale. The linked post says that New York State is spending vast sums of money just to house twelve inmates on death row, so obviously it would be cheaper to throw those twelve inmates into the existing prison system for life.

On the other hand, if we were to execute a thousand prisoners per year then it would start to get a whole lot cheaper. For instance, I'd be in favour of executing the scrote that stole my bike.