I just did a quick Google search on execution vs. imprisonment. I didn't have time to read the whole page, but here are some stats I found.

A Duke University study found... "The death penalty costs North Carolina $2.16 million per execution over the costs of a non-death penalty murder case with a sentence of imprisonment for life." ( The costs of processing murder cases in North Carolina / Philip J. Cook, Donna B. Slawson ; with the assistance of Lori A. Gries. [Durham, NC] : Terry Sanford Institute of Public Policy, Duke University, 1993.)

"The death penalty costs California $90 million annually beyond the ordinary costs of the justice system - $78 million of that total is incurred at the trial level." (Sacramento Bee, March 18, 198.

"A 1991 study of the Texas criminal justice system estimated the cost of appealing capital murder at $2,316,655. In contrast, the cost of housing a prisoner in a Texas maximum security prison single cell for 40 years is estimated at $750,000." (Punishment and the Death Penalty, edited by Robert M. Baird and Stuart E. Rosenbaum 1995 p.109 )

"Florida spent an estimated $57 million on the death penalty from 1973 to 1988 to achieve 18 executions - that is an average of $3.2 million per execution."
(Miami Herald, July 10, 198.

"Florida calculated that each execution there costs some $3.18 million. If incarceration is estimated to cost $17000/year, a comparable statistic for life in prison of 40 years would be $680,000."
(The Geography of Execution... The Capital Punishment Quagmire in America, Keith Harries and Derral Cheatwood 1997 p.6)

Figures from the General Accounting Office are close to these results. Total annual costs for all U.S. Prisons, State and Federal, was $17.7 billion in 1994 along with a total prison population of 1.1 million inmates. That amounts to $16100 per inmate/year.
(GOA report and testimony FY-97 GGD-97-15 )

http://www.mindspring.com/~phporter/econ.html