This is a purely political decision, not an inherent cost of jailing.
Your number comes down to $100k per person per year. That’s just insane. Many families earn less than that (post-tax)!
And obviously jail is supposed to be cheaper than non-jail life in the first place, because you’re not paying for luxury, just food, (cheap) rent and security.
That's not nearly as bad as I was expecting considering that for every 1-2 prisoners there's a ~$100k employee.
Or consider institution GED classes. You might say, those can easily go on the chopping block to save some money. But then you end up with inmates who are released without a high school diploma and, lacking educational opportunities, are more likely to return to crime. Then they go back into the prison system where they use more state resources than if they had just been given education in the first place. It's easy to imagine scenarios in which programs like that are worthwhile in the long term purely for fiscal reasons even if you care 0% about the welfare of criminals themselves.
Low key jobs program at the expense of taxpayers IMO.
Add admin staff etc, and the numbers escalate quickly.