Strangely, it actually costs the state more money to execute a prisoner than it does to keep them in prison. Death row inmates are guaranteed a very long and thorough judicial process by the constitution to minimise the chance of a wrongful execution, and these processes typically involve the most experienced and expensive lawyers and frequently use a lot of DNA testing, which is very expensive.

Many death row inmates also remain on death row for decades during this extended trial and appeal process, spending most of their time in special buildings separated from other prison populations, which require additional upkeep and more guards. In Texas for example, it costs an average of 2.3 million dollars - this is about 3 times more than is would cost to imprison someone in a maximum security prison for 40 years. It varies from state to state, but in every case it is more expensive to execute someone than it is to give them life in prison. In Maryland it costs an average of $3 million ($1.9 million more than it would cost to incarcerate them for life without parole), and in Pennsylvania this website suggests they've spent $350 million on the death penalty over a period of time where only 3 inmates actually ended up being executed.