The Commission of Investigation into Mother and Baby Homes has cost the State nearly €13.5 million so far.
The inquiry's final report is due to be published at the end of next month, after significant delays.
This investigation was set up in 2015 to look into Mother and Baby Homes and a small number of county homes between 1922 and 1998.
It's looking into living conditions and treatment of mothers and children, along with death rates, post-mortem practices and other issues.
According to details released to Newstalk under the Freedom of Information Act, the commission has cost over €11 million so far.
This includes nearly €9.9 million on administration and almost €1.2 million on legal fees.
The Department of Children has spent another €2.46 million on supporting the work of the commission and responding to its interim reports.
Christine Hennessey is from children's charity Barnardos.
She said: "We understand it's a very difficult and detailed process: many groups need to be consulted with and many stories need to be told.
"We'd hope with that much money being spent on the commission, that similar attention will be given to the ongoing support needs in terms of services for these women when the report finally comes out."
The department says the commission is due to deliver its final report by the end of next month.