Nationalist MSP Alex Neil has called on the Scottish Executive to come clean over the cost of running Scotland's only privately operated prison.
Kilmarnock is Scotland's only privately operated prison
The executive has always refused to give information about the cost of Kilmarnock Prison, saying that it was commercially confidential.
The Scottish National Party MSP's own research suggested it costs £17,602 per prisoner per year at Kilmarnock.
But that cost did not include mortgage costs for the prison building, he said.
The Scottish Prison Service (SPS) said the average cost for Scotland was £32,675.
However, Mr Neil said there were some old buildings in the prison estate which were very expensive to maintain.
He said that Kilmarnock, which opened in 1999, ought to benefit from low running costs of a new building.
Mr Neil said the lack of any official information on the costs of the private prison made it difficult to compare it with other jails.
But he said there were publicly available clues which helped him estimate what Kilmarnock Prison cost.
He said that notes to the Scottish Prison Service's annual report for 2003-2004 revealed that there was £10.4m for Public Private Partnerships, of which Kilmarnock was the only significant one with which the SPS was involved at the time.
He arrived at the £17,602 figure by dividing this cost by 596, the number of reported prisoner places at Kilmarnock in 2004.
A spokesman for the SPS said that it could not confirm Mr Neil's figures.
However, it said that the figure he quoted would mean that Kilmarnock cost almost half as much as prisons of a comparable size and security level, such as Edinburgh, Shotts and Glenochil.
Mr Neil said: "I am writing to the auditor general for Scotland to ask him to carry out a truly independent inquiry into the costs of Kilmarnock Prison and to compare these on a like-for-like basis with the costs of running our publicly-run prisons in Scotland.
He added: "Furthermore the secrecy surrounding the contract to run Kilmarnock Prison needs to be ended.
"This is public money which is being wasted on a private prison, which as well as being costly to run has one of the worst performing records of any prison in Scotland."