It costs as much in California, because of activism that deliberately makes it more expensive than necessary and then claims that it is too expensive to make sense. California could strike a deal with Kentucky, Oklahoma, or Nevada to send their prisoners there at half the price it costs to incarcerate them in California, and save billions while allowing the destination states to make very nice profit as well. Of course, this is not going to happen, because high prison costs in California is a feature, not a bug.