Although, in the last decade, many companies have saved up to 75% in labor costs by outsourcing to nations like China, prisoners in the US have legally been paid as little as $0.16/day. This results in $41.60 per laborer per year in expenses.
As of 2010, working couples paid an average of $500 in taxes that were used for corrections, cumulatively, for federal, state and local institutions. If the prison system was a closed ecosystem, that tax burden could be carried by the incarcerated themselves, loosening taxes on struggling families.
Studies have shown that filling an inmate's day with work keeps their mind occupied, lowering instances of adverse activity and violence. Teaching them skills they can apply after release also lowers the rate of recidivism, contributing to rehabilitation for society.