Market rates are not arbitrary numbers that exist in a vacuum. All economic actors contribute to the market, and they are the result of competition and how low the producers are able to get and still make a living. Some are a bit higher, some are a bit lower. If the prison can always go as low as any other producer and out-compete farms that can't use slave labour to stay artificially competitive, then it will eat the market share of these farms, which will progressively go bankrupt, and the only thing that will stop the prison from eventually owning the entire market is the number of people that get convicted of some crime and jailed there.