The police security operation at Stormont is costing £8,000 a week, the assembly comission confirmed.
There are seven officers in the Assembly during plenary sessions.
The bill is £413,000 a year, the Commission which runs the building revealed.
Loyalist killer Michael Stone was jailed for 16 years for trying to murder Sinn Fein's Martin McGuinness and Gerry Adams at Stormont.
Stone attempted to enter the building during a key debate on Northern Ireland devolution in November 2006 but was wrestled to the ground and disarmed by security officers.
Assembly Commissioner Sean Neeson, who helps run the building, said: "The Michael Stone incident did impact very much upon the number of police personnel who have been taken on board to secure this building and to look after the safety not just of members but of members of the public as well."
There is a dedicated visitors' search area close to the entrance to parliament buildings and a PSNI presence inside the building. Total numbers vary from seven to three.
Paul Maskey, Sinn Fein chairman of the Assembly's Public Accounts Committee, said they needed to monitor the expenditure.
"If there's a possibility that it can be cut then it should be cut," he said.
"It is £413,000 a year so it is something, I have to say, it is a large amount of expenditure."
© Press Association