The prison commissary is a company store. The prisoner may not buy from any other source and she or he must pay whatever the commissary decides to charge. In theory, the prison commissary is operated for the benefit of the Inmate General Welfare Fund. For example, recently over a million dollars from the fund was siphoned off so that the DOC could buy into a television scheme for the guards to get college degrees. The money which prisoners spend in the commissary, is either donated to them by their families and loved ones, or it's earned by the prisoner by doing menial labor within the prison.
Many prisoners receive occasional donations from their families loved ones.
In effect, loving families, almost all of whom are from the very most impoverished part of our society, support prisoners. The DOC has a specific strategy to wring as much money as possible out of the impoverished families. In many cases, it's the families' money which is spent in the prison commissaries. Prisoners' families are already greatly overtaxed by the prison system. The telephone system alone is a racket which bleeds families for many millions of dollars each year.