I'm generally in line with Dana Jean, guido, and Tery on the subject. Which means I do not think I have any kind perfect answer, because the circumstances of each case are different. Since we live in a finite world with limited resources, I would point out some of the costs in a different manner. The time from booking to execution continues to grow. I understand that costs are now $50,000+ per year to house an inmate. If they are there for 20 years (which is getting more common) that is $1,000,000 in expenses (and the longer they stay the more it piles up). Notice the chart on this site.
http://www.deathpenaltyinfo.org/time-death-row

Is it better to house and feed the innocent hungry, or house and feed killers, many with no remorse for their crime? I have a feeling if budget cuts continue, we, as a society, may have to revist this question with a different perspective. And if not death, is there something more creative as an answer, as sometimes is written about in near future sci-fi books.