The price of keeping suspected terrorists at a detention center in Guantanamo Bay may be even higher than officials had estimated. According to a new report from the US Department of Defense, America will have spent $5.24 billion (pdf) on the prison camp between 2002, when it was opened, to 2014.

Guantanamo Bay is already said to be the most expensive prison in the world. Yet even the latest figure from the US Defense Department might not be high enough. The Miami Herald points out that total costs are likely to be higher because the Defense Department doesn’t take into account the facility’s $13.5 million headquarters built in 2004. It also doesn’t include costs for “Camp 7“, a compound for detainees formerly kept by the Central Intelligence Agency, like Khalid Shaikh Mohammed, the self-proclaimed architect of the September 11 attacks.

This year, operating costs will hit $454.1 million. By comparison, maximum security prisons in the US cost between $60,000 to $70,000 a year to maintain.

The priciness of Guantanamo is partly about location. Operating a US naval base on the Cuban coast means almost everything has to be shipped, including food, construction material and personnel. Judges and lawyers have to be flown in on military aircraft.

US president Barack Obama recently returned to his earlier promise to shut down the center, which he says is motivated in part by the cost of maintaining the prison. (Others criticize the fact that prisoners are kept for years before being charged or put on trial). But there’s no consensus on what to do with the 166 inmates currently housed there, some of whom US lawmakers say are “too dangerous to release.” Until there is an alternative, the detention center must stay open, conservative lawmakers say. That seems to be the case, for now: The Department of Defense is calling for the facilities to be upgraded. That would cost another almost $200 million.