I was so ready to just downvote for implying criminals don't deserve rights and move on, but something about your argument does give me thought and I kinda wanna explore this more, especially since you're working in this field yourself. Apologies if this is kinda ranty, I'm processing my thoughts and why the comment stuck with me as I go.

When people are put in jail, it costs money, like you said, it's not free, so what do taxpayers expect to get out of that money? Depending on the answer to that question, yeah, it seems fair to think of manual labor as a way to offset the cost (though I doubt the taxpayer is seeing the benefits of those offset costs). I'd like to think in an ideal world however, what is being paid for is rehabilitation; turning criminals back into contributing members of society, in which case, their future productivity would ideally offset the costs to society of jailing them. I think there are enough stories of violent offenders who go on to be lawyers and such to show this is possible and what society should strive for, though I know it's not easy and may not be possible in all cases. In this case, forced, manual labor would probably be counterproductive, but arguments could be made for voluntary or even paid labor that contributes to rehabilitation.

Outside of rehabilitation, there's also the simple idea that jails are just paid to keep these people off the streets, in which case the consideration of cost is just, how badly do you not want these people in society? Following that idea, forced labor kind of makes sense, they're just in jail to be off the streets, might as well make them useful while they're removed from society, but then the argument is how much labor is ok?

Then again, if the idea is strictly punitive, any amount of forced labor becomes justified since the idea is that they are in there to suffer and serve penance. In this case I suppose the only consideration is whether the amount of penance in the form of labor fits the crime.

I'm just thinking out loud here, I don't really know anything about this topic for certain. My background though is in special education, and I know from my early studies that whenever it's tested, a lot of inmates turn out to have some sort of diagnosable learning difficulty, so I feel that the existence of jails to a large extent is a failure of society to support vulnerable people. At the same time, I do recognize that there are people who we really don't want to be part of society and whom rehabilitation might not be possible, but then if I let my thoughts go down that direction the logical conclusion seems to be the death penalty? And that's not something I'm really for, especially since there are already so many cases of wrongful death penalties.

tl;dr, I guess we really have to know what we want as the goals for jails as a society, communicate that clearly, and from there we can talk costs and the potential usefulness of forced labor.