"Substance-involved people have come to compose a large portion of the prison population. Substance use may play a role in the commission of certain crimes: approximately 16 percent of people in state prison and 18 percent of people in federal prison reported committing their crimes to obtain money for drugs.21 Treatment delivered in the community is one of the most cost-effective ways to prevent such crimes and costs approximately $20,000 less than incarceration per person per year.22 A study by the Washington State Institute for Public Policy found that every dollar spent on drug treatment in the community yields over $18 in cost savings related to crime.23 In comparison, prisons only yield $.37 in public safety benefit per dollar spent. Releasing people to supervision and making treatment accessible is an effective way of reducing problematic drug use, reducing crime associated with drug use and reducing the number of people in prison."

Source

Justice Policy Institute, "How to safely reduce prison populations and support people returning to their communities," (Washington, DC: June 2010), p. 8.