You may be surprised just how much death row costs. Part of being ‘absolutely sure’ is not executing until all possibilities of appeal and such are exhausted, and well, if your scheduled to be killed most people will do whatever they can to delay it for as long as possible, or even better get it overturned. Some end up on death row for decades.

Some data here: https://deathpenaltyinfo.org/policy-issues/costs/summary-of-states-death-penalty