This 2008 report evaluates California's Proposition 36. 

From the report:

This Proposition 36 (Substance Abuse Crime Prevention Act) 2008 evaluation report has four sections. The first section describes the characteristics of clients and of treatment utilization and outcomes in Proposition 36. The second section provides information on two special sub-populations in Proposition 36: high risk/high cost offenders and homeless mentally ill offenders. The third section provides preliminary evaluations of several promising practices with potential to improve treatment program performance and client outcomes in Proposition 36. The final section examines re-offending outcomes, presents the findings of a cost offset analysis, then discusses additional measures that can be used to monitor performance and outcomes in Proposition 36.

• Study 1 extended the baseline and follow up periods used in UCLA’s earlier cost report from 30 months to 42 months. Here, costs for a pre-Proposition 36-era comparison group and for all first-year Proposition 36-eligible offenders found a net savings of $1,977 per offender (N = 61,609) over a 42 month period, yielding a benefit-cost ratio of nearly 2 to 1. In other words, $2 was saved for every $1 invested. 
• Study 2 used first year Proposition 36 participants who were referred to the program. Proposition 36 participants who completed treatment achieved a benefit-cost ratio of approximately 4 to 1 over a 42-month period, indicating that “completers” saved $4 for every $1 allocated. 
• Study 3 examined follow-up costs for succeeding year as the policy matured.  Over a 30-month follow up period, the costs for jail, probation, parole, and treatment have remained stable from year to year. Prison costs and costs for arrest and convictions have steadily declined over the first 3 years.  Two conclusions follow from the cost analyses: Proposition 36 substantially reduced incarceration costs and resulted in greater cost savings for some eligible offenders than for others.

Full report: Evaluation of Proposition 36: The Substance Abuse and Crime Prevention Act of 2000 (PDF | 1.23MB)

Department of Alcohol and Drug Programs, California Health and Human Services Agency. Evaluation of Proposition 36: the substance abuse and crime prevention act of 2000, 2008 report. University of California Los Angeles, Integrated Substance Abuse Programs. Urada, D., Hawken, A., Conner, B.T., Evans, E.M., Anglin, M.D., Yang, J., Teruya, C., Herbeck, D., Fan, J., Rutkowski, B., Gonzales, R., Rawson, R., Grella, C., Prendergast, M., Yih-Ing, H., Hunter, J. and Poe, A.