Ireland's most expensive empty field? Prison site now worth less than 10pc of €30m spent by Government
The controversial Thornton Hall site is now worth less than a twelfth of the €29.9m the Government paid for it in 2005.
A Valuation Office report for the Prison service put the worth of the site at just €2.4m earlier this year.
C&AG Seamus McCarthy criticised what he described as an "inadequate analysis" of the likely costs of developing a new prison there.
He found the costs were significantly understated at €150m in 2004. Just two years later this was revised upward to €525m.
Mr McCarthy also found the cost of addressing the overcrowding and facilities problems at the Mountjoy complex - one of the main drivers for Thornton Hall in the first place - were overstated.
Initially purchased in 2005 for a planned 1,400-place super prison, the north Co Dublin project never got off the ground due to the economic crash.
Although it is hoped that it could eventually become a much smaller prison development, a review group is currently examining the options for its use.
According to Mr McCarthy's report, some €50.6m had been spent on the project up to May of this year.
Additional land purchases cost over €2m, road access development cost €3.4m, site-preparations works amounted to more than €3m, while the provision of water, telecommunications and sewage services have cost €2.6m.
An additional €7.9m has also been paid out in technical, legal and financial fees, and close to €1m has been spent on landscaping and maintenance.
Mr McCarthy outlined how the need for Thornton Hall significantly diminished as improvements were made at Mountjoy.
The Mountjoy upgrading work cost €28.7m between 2010 and last year.
"Since 2010, many of the problems at Mountjoy have been dealt with at a much lower cost than that proposed originally to build a new prison on the Thornton site," the report said.