We spend substantially more on prisoners than they could ever hope to generate in labor.
After all, the average prisoner is not a diligent, sober, hardworking person trying to get ahead; how much economic value are you really likely to extract, even if you're evil?
Chattel slavery and prison labor may be distant relatives, but they're not siblings, and it's wrong (in a dozen different ways) to imply they are.
To make a long story short, in the US, you are and have always been one of two things: the exploited or the exploitor.