The prevalence of serious mental illness in prison is much lower than that. Psychotic illness runs in a varying rate between about 5% and 8% of the standing prison population; bipolar disorder, around 2% to 3% of the prison population; and current major depression, maybe 15% to 20%, depending upon which study you look at. So the total is about 25% or so.
Between 15% and 25% have one of those diagnoses. Of the people who have one of those diagnoses and are in prison, more than 90% will also have a drug addiction or alcohol addiction problem. Once you have a mental illness, your co-morbidity is very high against the rest of the prison population.