@ "Sinisterspade" (page two) - the death penalty actually costs more than life imprisonment. The process of conviction, then the appeals, so on and so forth, add up to more in lawyer's fees than the cost of keeping a man in prison for his life. If we're talking practicality, imprisonment is cheaper.
Of course, you have to factor in the cost of building new prisons all the time thanks to ever more ridiculous laws. Whether or not prisons eventually pay for themselves as opposed to the cost of executing (or more accurately, the cost of being allowed to execute) someone, is unknown to me.