Political considerations played no part in the decision to drop charges in the "Stormontgate" affair, the Public Prosecution Service (PPS) has insisted.
Devolved government collapsed in October 2002 after police raid
Charges related to allegations of IRA spying against three men were dropped last month "in the public interest".
One of the men, former Sinn Fein Stormont official Denis Donaldson, later admitted he had been a British agent since the mid-1980s.
The PPS said it could not confirm what the two-year case cost the taxpayer.
In response to a series of questions put forward by the BBC, the service said it did not hold information on the individual costs of cases.
It said this was "only one of the large number of cases the PPS deals with in the course of a year".
Legal sources told the BBC that while costs would have built up over the two years, the fact that it did not go to a full trial would have avoided considerable expenditure.
The three were arrested following a police raid on Sinn Fein's offices at Parliament Buildings on 5 October 2002, when documents and computer discs were seized.
Following the arrests, the Reverend Ian Paisley's Democratic Unionists and the Ulster Unionists, led at that time by then First Minister David Trimble, threatened to collapse the executive with resignations.
The British government then suspended devolution in the province, embarking on direct rule for the last three years.
However, the "Stormontgate" case collapsed on 8 December 2005 when charges against the three men were dropped.
Despite pressure from unionists and the nationalist SDLP, the PPS has consistently refused to be drawn further on why the charges were dropped.
One week later, Mr Donaldson, who headed the party's administration office at Stormont before his arrest, admitted he was a paid British agent for two decades.