Yeah.. according to the stats in this post, about 1.9 million people are behind bars.

11 billion a year between 1.9 million prisioners is about 6k per year per prisoner.

So that's about $500 a month per prisoner. Which is not as big of a number.. and according to the article, the prisoners still get paid a fraction of it.. it's just that 80% is used "for room and board, court costs, restitution and other fees, including the construction and maintenance of prisons".