This is a meta-analysis that was published in 1999. [1]
The authors analysed the impact of psychological interventions on the use of medical services by examining the outcome of 91 studies published between 1967 and 1997. Psychological treatments included psychotherapy, behavioural medicine, and psychiatric consultation. Psychological interventions in hospitals and other settings reduced the length of stay by 2.5 days and overall health care costs per patient by about 20 per cent.
1 Chiles J. The impact of psychological interventions on medical cost offset: a meta-analytic review. Clin Psychol Sci Pract 1999;6:204–20. doi:10.1093/clipsy/6.2.204