Estimates for additional costs aggregate to something like an extra billion on trials for capital punishment. That's $1 billion NOT going to schools, prevention, cops etc. Sure it doesn't seem like a lot but you figure we could simply throw these scumbags away to rot for life and be a billion dollars richer, well, that my friend is logic. Furthermore, per capita crime doesn't show any strong trends based on states that have capital punishment and states that don't. States without it have varying rates and states with it have varying rates. What the trends DO suggest is that states with terrible economies tend to have higher rates of violent crime, but really, that's not news to anyone.
The system in the US does not make a whole lot of logical sense. High cost, low execution rate, no evidence of deterrence. And we don't even execute those who steal billions.
While in theory the idea isn't bad, in practice America's system is totally screwed up. You're more likely to be killed on the street selling crack then you are on death row.