It is a nonsense to argue for or against a punishment on the grounds of its cost, because justice is something we should be prepared to pay for (and on this point, defendants should always have access to decent lawyers). Nevertheless, many Americans wrongly believe that the death penalty is cheaper than execution - this shows how wrong they are:
The death penalty is hideously expensive in the US. From the point of arrest to execution, a single death penalty case can cost $1-$3 million, according to various state governments' estimates. Some studies have put the cost as high as $7 million a case. Cases resulting in life imprisonment generally cost around $500,000 each.
Since the late 70s, the US has spent $1 billion on the death penalty (additional to the cost that would have been occurred if these had been non-d.p. cases), it is estimated. In Florida, taxpayers spent $2.3 million per execution. In New York state, the Department of "Corrections" estimated that bringing back the death penalty would cost $2 million per case, or $118 million anually. They brought it back anyway, and sharp cuts were made to public funding of education and health in order to pay for it.
In 1991, New Jersey spent $16 million on the death penalty. The following year, it laid off 500 police officers because it could no longer afford their salaries.
The two-phase conviction, post-sentencing review and other safeguards insisted on by the Supreme Court make the death penalty very expensive. Yet these safeguards are essential: otherwise, the number of innocent people dying (which can never be reduced to zero) would rise very considerably. And yet making people wait so long to be executed is itself grossly inhumane. And even with the costly safeguards, innocent people continue to be convicted, to spend many years in prison in appalling conditions, and sometimes to be executed. Texas spends $2.3 million on each death penalty case - 3 times the cost of keeping someone in prison for 40 years.