USA Median yearly Income: 43550$ (source). nearly half of the American people (who worked year round, full time jobs) can't afford to pay back the state for staying in prison.
do you really expect a guy in prison to be able to make that kind of money, considering he gets 0.25$ an hour?
which rounds off to 160,000 working hours (number of hours in a year? 8760) even if the 0.25$ is incorrect, unless it's almost 15$, it's not going to happen.
prisons, by definition, can't be self supporting. slave camps, on the other hand, can be.