I think while proposals like this may sound good, they along with imprisoning people for non payment of child support bring this country dangerously close to having "debtors prisons" a quaint english custom that the founding fathers were deadset against.
What happens if the prisoner can't or simple refuses too pay for their imprisonment? Do they keep them locked up while the fees continue to mount?
What about those convicted of a crime...and fined? Sure, if you have assetts or a hot-shot daddy, you pay the fine and walk. Those that can't stay in jail to work out the fine.
Me, I think if one is convicted, they should serve time only, each and everyone, courtesy of the taxpayer.