The cost of incarceration is high, yes, but there is a price to be paid for not incarcerating people who need to be incarcerated, and the people who pay that price are most often the poor. The poor are disproportionately the victimes of crime.
The rich can insulate themselves, in their exclusive neighborhoods, gated communities, and with their expensive alarms systems. The poor have to count on the state to fund the aggressive law enforcement and long prison sentences that help keep them safe.
When prison sentences lengthen, human suffering is reduced.