“The going rate” will be low, because of market dynamics. Either you need a prisoners' minimum wage, or you need to not do that; people do still have rights even if they've committed crimes.
Put them in education, give them food and shelter, and they'll be better able to contribute to society when they get out.
They should have the option of school or work, with a minimum GPA required to stay in school. The biggest issue will be supervision of the grading to ensure it's not being abused either to punish people who are doing well or to let people off who aren't actually doing their work.
Put them in education, give them food and shelter, and they'll be better able to contribute to society when they get out.