WASHINGTON - It's been dubbed the most expensive prison on Earth and U.S. President Barack Obama cited the cost this week as one of many reasons to shut down the detention centre at Guantanamo Bay, which burns through some $900,000 US per prisoner annually.

The Pentagon estimates it spends about $150 million each year to operate the prison and military court system at the U.S. Naval Base in Cuba, which was set up 11 years ago to house foreign terrorism suspects. With 166 inmates currently in custody, that amounts to an annual cost of $903,614 per prisoner.

By comparison, super-maximum security prisons in the United States spend about $60,000 to $70,000 at most to house their inmates, analysts say. And the average cost across all federal prisons is about $30,000, they say.

The high cost was just one reason Obama cited when he returned this week to an unfulfilled promise to close the prison and said he would try again. Obama also said that the prison, set up under his Republican predecessor George W. Bush and long the target of criticism by rights groups and foreign governments, is a stain on the reputation of the United States.

"It's extremely inefficient," said Ken Gude, chief of staff and vice president at the liberal Center for American Progress think tank, who has followed developments at Guantanamo Bay since 2005.

"That ... may be what finally gets us to actually close the prison. I mean the costs are astronomical, when you compare them to what it would cost to detain somebody in the United States," Gude said.

The cost argument could be a potent weapon at a time of running budget battles between Obama and the Republican-controlled House of Representatives, and of across-the-board federal spending cuts that kicked in in March. The "sequestration" as it is known, is due to cut some $109 billion in spending up to the end of September and has cut government services small and large.

Just one inmate from Guantanamo, for example, is equivalent to the cost of 12 weeks of White House tours for the public - a treasured tradition that the Secret Service says costs $74,000 a week and that has been axed under sequestration.

A single inmate is also the equivalent of keeping open the control tower at the Northwest Arkansas Regional Airport for 45 months. That control tower, another victim of cuts, costs $20,000 per month to run.

The $900,000 also matches the funding for nearly seven states to help serve home delivered meals to the elderly. Sequestration has cost Meals on Wheels a median shortfall of $129,497 per state, the organization says.

Or measured in terms of military spending and national security, the cost of four inmates represents the cost of training an Air Force fighter pilot - based on the Department of Defense's figure of $3.6 million per pilot.

WHY THE HUGE COST?

The huge cost of running the prison and judicial complex stem from its offshore location at a 45-square-mile U.S. Naval Base on the southeastern coast of Cuba. Because ties between the two countries are almost nonexistent, almost everything for the facilities has to be ferried in from outside.

When the military tribunals are in session, everyone from judges and lawyers to observers and media have to fly into Guantanamo on military aircraft. Food, construction materials and other goods are shipped in from outside, experts say.

But despite the high cost of the camp, and despite the fact that Republicans traditionally demand belt-tightening by the federal government, a Republican aide with the House of Representatives Armed Services Committee said there was little point in asking if the price was worth it because "there isn't an alternative at the moment."

"No one has any particular affection for Guantanamo Bay, but no one has come up with a practical solution that's better," the aide said.

Obama needs to produce a plan for what to do with the detainees at Guantanamo "who are too dangerous to release," Representative Buck McKeon, the chairman of the House Armed Services Committee, said in an opinion piece in USA Today this week. "Until a better solution is offered, at Guantanamo they must stay," he wrote.

Among current inmates, nine have been charged with crimes or convicted, 24 are considered eligible for possible prosecution, 86 have been cleared for transfer or release and 47 are considered too dangerous for release but are not facing prosecution.

But until now, worries about security have prevented the idea of transferring some or all of the inmates to the United States from getting much traction.

Obama pledged to close the prison within a year after first taking office in January 2009 but his efforts ran aground, partly because of congressional opposition, from both Republicans and some in his own Democratic Party, to transferring prisoners to the United States.

Inmates started a hunger strike in February that has swelled to some 100 prisoners and has led to force-feeding of 23 of the prisoners. With the camp back under a critical spotlight, Obama told a news conference on Tuesday he would renew efforts to shut it down. He has an array of options, some of which would be more achievable than others.

Gude said it was difficult to figure out how much the United States has spent overall on Guantanamo detention facilities since it began housing prisoners there in 2002 because administrations only recently have been noting the expense in a budget line item.

"I don't know if I've ever seen an estimate but it is certainly more than $1 billion by a comfortable margin, I would say, probably more than $2 billion," Gude said.

Above the annual operating cost, capital spending on the prison could rise again if the Pentagon receives the funding it says it needs to renovate the place.

General John Kelly, the head of Southern Command, which is responsible for Guantanamo, told a House of Representatives panel in March that he needed some $170 million to improve the facilities for troops stationed at the base as part of detention operations. Kelly said the living conditions were "pretty questionable" and told the panel, "We need to take care of our troops."