Look at what Aleksandr Solzhenitsyn wrote in The Gulag Archipelago about how Gulag camps proved economically profitable.
The reason why the camps proved economically profitable had been foreseen as far back as Thomas More, the great-grandfather of socialism, in his Utopia. The labor of the zeks was needed for degrading and particularly heavy work, which no one, under socialism, would wish to perform. For work in remote and primitive localities where it would not be possible to construct housing, schools, hospitals, and stores for many years to come. For work with pick and spade–in the flowering of the twentieth century. For the erection of the great construction projects of socialism, when the economic means for them did not yet exist.
The Gulag Archipelago, Volume 2, pp 578-579
The camps were uniquely profitable in terms of the submissiveness of the slave labor and its cheapness–no, it was not just cheap, it cost nothing, because in antiquity money did have to be paid for a slave, whereas no one paid anything to buy a camp inmate.
The Gulag Archipelago, Volume 2, p 581