If I put the numbers together correctly from early 90s: inmates worked ~30 hrs?/week at ~$0.25/hr (or was that /day?) for an income of <$7.50/week. If that's 1% of potential earnings outside, that's $750/week. I guess you're in the ballpark, but with that low pay comes three meals a day and a bed, however crummy those are.
(There may be a reason in this somewhere why most of the homeless are not vying to become prison inmates.)
.....................and FREE Health Care ;-)