Correctional facilities are under increasing pressure to respond to the treatment needs of mentally disordered offenders during their incarceration and to arrange for treatment post release through reentry planning. This paper constructs cost estimates for three different reentry investments using data on the population (n = 2715) of male mentally disordered inmates in New Jersey prisons. Different assumptions are used on the distribution of treatment needs and criminal characteristics within the population, ranging from equal treatment needs and placement difficulties to different treatment needs and difficulties accessing community-based resources as a consequence of their past criminal behaviors. The costs of these investments range from approximately $700,000 to $934,000 per year for roughly 500 released inmates. Reentry planning, as a social investment, protects the outcomes produced by correctional health care during incarceration and protects the public from future crime associated with untreated mental illness. (Contains 5 tables.)