Spiritually speaking, why does the death penalty cost so much?

It is because of automatic appeals. Since the US justice system is more about "winning" than it is about justice, that's a good thing. Some years back (7? 10?) a Florida death row inmate had to fight the district attorney to get access to the DNA evidence in his case. Even after the DNA cleared him and it turned out the guy was not guilty of anything, the state district attorney was still fighting to have him executed, saying guilt or innocence didn't matter because the man had been found guilty by a jury of his peers! With crap like that going on I really think we should eliminate the death penalty completely. The automatic appeals all the way to SCOTUS is just a bit of protection - assuming the DA in your case doesn't "accidentally" destroy the evidence before your execution so it can not be tested by the defense (which has happened, btw). It really does need to be there and even with that in place, there are a few people that have been executed in the USA that most likely were not guilty - but the relevant evidence has been destroyed by the DA in their cases.