The State spent €700,000 each on keeping five young offenders in the State's only secure detention school for girls last year.
The total cost to the State of detaining 54 young people in the five child-detention schools in 2006 was just under €25 million, an average of almost €450,000 a head, according to figures released to The Irish Times.
Rising costs and falling occupancy levels in the schools are contributing to the increasing costs, which are almost five times the cost of keeping an adult in prison.
In spite of frequent complaints at the Children's Court and elsewhere that there are not enough places for young offenders, the figures indicate that the facilities are being under-utilised.
The highest cost last year was for Oberstown school for girls at an average of €699,103 per child. This compares to €575,622 in 2005 and €454,000 in 2004.
The second highest cost in 2006 was for the Finglas Child and Adolescent Centre at an average of €486,506 each. The cost was less than the €497,960 figure for 2005, and €507,407 in 2004.
The average cost per child in Trinity House last year was €396,016, compared to €335,949 in 2005 and €362,000 in 2004.
The cost for Oberstown boys' school last year was €329,956, compared to €364,278 in 2005 and €282,000 in 2005.
Figures were unavailable for the cost of detaining children in St Joseph's special school in Clonmel in 2006, but the cost in 2005 was €222,377.
The figures released by the Department of Justice also show that the detention schools are not operating at full capacity.
In 2006 the State's detention schools, outside of St Joseph's, had an operational capacity of 67 but an average occupancy of 54.
In 2005 the operational capacity was 74, and the average occupancy was 52.
In a statement to The Irish Times on costs, the department said: "While the costs associated with these schools are acknowledged as being high, the priority of the detentions schools system is to divert the child from further offending, and provide the training and education to him or her in becoming a productive member of society rather than reducing the average cost of detention."
On March 1st responsibility for Finglas Child and Adolescent Centre, Trinity House, and Oberstown schools for boys and girls was transferred from the Department of Education to the Irish Youth Justice Service in the Department of Justice.
The department said yesterday that the detention of young offenders in child-detention schools was very different from the detention of adult offenders in prison. "It is based on principles of care and education aimed at promoting the child's reintegration into society."
It said the low number of young people detained in schools last year reflected the philosophy of the Children Act 2001.