http://bridgemi.com/2015/08/43000-michigan-prisoners-who-should-we-cut-loose-first/
The prison population is now about 43,000 and is down from a high around 60, 000. The cost per prison for a year of incarceration is about $35, 000. The Michigan prison system budget is at $2 Billion.
There is a lot of demand to reduce the population but this article talks about who do you let out first and how do you reduce the future flow into our prisons.
Well worth a read.