A slightly different perspective:
A few years ago while doing some research - most of which I've forgotten - into this, I ran across and interesting item.
Seems that due to the required legal wrangling and mandatory appeals for a capital sentence, it is actually almost an order of magnitude more expensive to execute someone instead of imprisoning them for life.
What is the point of the sentence? Is it to punish the convicted for their crime? If so, is the punishment to fit the crime? If yes, then there should be a deterrent factor in punishment for the convicted as well as for others. There can be no doubt that there is a deterrent factor for the convicted. You can't get much more deterred than dead. However, every piece of research into this has indicated that there is no deterrent factor for others.
Is the point to remove a hazard to society? If yes, then permanent imprisonment does the same thing.
Is it to rehabilitate where possible, punish, and remove a danger to society? If this is the logic, the system is doing a very poor job.
You really can't touch the concepts of capital punishment without also dealing with the concept of punishment and the basis of the criminal justice system.
Don't know about other states, but Texas has one out of 20 people currently involved in some form of prison / parole / probation. One of the biggest growth businesses in the state is PRISONS.
So, the conclusion I make on this is that what we are doing doesn't work.