Small note, I personally think cheapest way to do executions is with a cattle spike. You get these pneumatic powered spike guns that are shot into the head of cattle when they are slaughtered. Its reasonably humane because one cannot feel pain if the brain is destroyed. Death is basically instantaneous, and it is re-usable.
I feel is it more humane than a guillotine, and cheaper than execution by firearm.
It should be noted that I am personally against the death penalty, but if you are so dead set about having it, at least you can make it efficient.