Most if not all of the debtors' prisons were privately run, by contractors, who made a pretty good living by (over)charging their "guests" for rent and food.

So the debtors got even deeper into debt while they were in jail :-)

But that's a good idea, charging prisoners for their keep. I think they should do that today. And charge them also for any damage or expense incurred in the course of their crimes.

If criminals knew in advance that they would have to pay back every penny they have cost the taxpayer and their victims, for police time, cost of the operation to track them down/arrest them, cost of any damage caused, value of stolen property, etc., maybe that would deter some of them from committing crimes in the first place.

Hit 'em in the pocket - that's more effective than anything else these days.