So because doing X would only solve x% of the problem (without significant cost, as noted by another poster) for x < 100, we shouldn't do it? That's an absolutely insane argument. Apply it to business, software development, or whatever field you work in. The real key here is that commuting every death sentence to life in prison would cost almost nothing. So if it saves even one life, there isn't even a callous cost-benefit argument in favor of the death penalty! I myself am swayed by the moral arguments, no cost-benefit needed, but that's just me.