Keeping a prisoner on death row until execution is actually more expensive then just having the prisoner spend the rest if his/her life in prison without being on death row.
To keep a prisoner on death row the cost can vary because it depends on how long the prisoner is on death row before execution.
Some prisoners stay on death row from 10 years to 20 years and a few have stayed on death row for as long as 40 years before they were executed.
Keeping a prisoner on death row costs between $90,000.00 to $150,000.00 per year which is a lot.