$4,000,000,000 spent on capital punishment cases in California from 1980-2012 for a total of just 13 executions. This study http://digitalcommons.lmu.edu/llr/vol44/iss0/ provides evidence that between 1980 and 2012, California spent $4 billion administering death penalty cases while actually executing just 13 individuals.
Don't get me wrong, if the system was different and we weren't condemning innocent people left and right I wouldn't mind it that much. Personally I am against the death penalty just because life in prison seems a far worse punishment than the easy out that is death (I'd be lying if I said I didn't enjoy the "botched" executions that occurred in the last few months).
In the end though the priority is taxpayer money, and capital punishment is expensive as hell. Most people I know who support the death penalty think "this horrible person shouldn't be given free room and board for the rest of their life on taxpayer money" which I agree with, but in the end it is far cheaper to just serve life in prison.
It takes a lot of expensive judicial process and paperwork to legally kill someone.