The cost of looking after a patient at Mount Carmel Hospital varies between €76 a day and €654, according to Health Minister Chris Fearne.
He said that the cost depends on the level of treatment and care that they require.
He was responding to a parliamentary question posed by MP Mario Galea.
There are approximately 400 patients at any one time at the mental health hospital, but there is a shortage of staff: in certain wards, there is only one nurse for between 30 and 40 patients.
In February, it was reported that there were 22 patients supposed to be under constant watch.