The price per prisoner isn't the relevant metric. Total price to society, from the cost of maintaining and manning prisons, from the cost of lost human connections with those imprisoned, and from lost productivity from those imprisoned, is a relevant metric. A prison system with a 36% 2-year recidivism rate (United States) is a failure when compared to a prison system with only 20% 2-year recidivism rate (Norway), no matter how cheaply it may be managed.

The goal of a functioning prison system is to manage itself out of existence.