As the debate flares anew over whether to close the detention center at Guantanamo Bay, supporters of the prison camp are pointing to an often forgotten factoid: Gitmo is one of the best real estate deals the U.S. taxpayers have ever seen.

Under a treaty signed more than a century ago, the United States technically pays Cuba a little more than $4,000 a year for the land. That's roughly the monthly cost of an apartment in Midtown Manhattan. And it's a check the Castro brothers' government rarely cashes.

Sen. James Inhofe, R-Okla., among those lawmakers pushing for the camp to remain open, calls it a "good deal."

"Rent is only $4,000 a year, which Fidel Castro routinely refuses to cash ... out of protest,” the senator told FoxNews.com on Monday.

Guantanamo Bay is back in the headlines following the controversial trade of American Sgt. Bowe Bergdahl for five Taliban detainees at the camp, and following the capture of a sought-after Benghazi attack suspect -- whom some lawmakers, including Inhofe, say should have been brought to Guantanamo.

Stymied by Congress, President Obama still has not been able to fulfill his campaign promise to close the detention center. But the operation benefits from one of the cheapest land deals since the Louisiana Purchase.

The Naval Base at Guantanamo Bay is the oldest existing U.S. military base outside U.S. territory. Located near the northern tip of Cuba, it faces Haiti and Jamaica, and is located on what was once the Oriente Province.

Its history with the United States dates back to June 10, 1898 when it was used as a camp for the first U.S. troops to arrive in Cuba during the Spanish-American war. Five years later, the U.S. signed the deal with Cuba to lease 46.8 square miles of land in Guantanamo Bay for 2,000 gold coins – or $4,085 a year.

The Feb. 23, 1903, deal was made by President Theodore Roosevelt as part of the Cuban-American Treaty.

The agreement, which was effectively forced on the Cuban government, gave the U.S. Navy permission to occupy the bay. In return, the U.S. would pay the rock-bottom rent. The lease allowed the U.S. military to construct a permanent naval base on the site, which has been in regular use since the early 20th century. It was used as a prison camp for alleged Taliban and Al Qaeda terror suspects following the Sept. 11, 2001 terror attacks.

As cheap as the rental fee is, most years it's actually free.

Since 1959, Cuba has cashed only one U.S. rent check -- and this was done in error, Cuba’s former leader Fidel Castro later admitted in an essay. The Marxist political leader, who served as president of Cuba from 1976 to 2008, said in his “Reflections” column he had refused to cash the checks in protest of the “illegal” U.S. occupation of the land which he said was being used for “dirty work.”

The checks from the U.S., which Castro claimed were made out to “Treasurer General of the Republic,” have been sitting in a dusty desk drawer in Havana for years. During one television interview, he showed off the checks from the U.S. that he had squirreled away. Castro also claimed “Treasurer General of the Republic” is a position that doesn’t exist, in an essay published by the ruling Communist Party newspaper Granma.

While the cost of leasing the land comes cheap, operating the detention center is another matter.

A 2013 report by Carol Rosenberg, the dean of the Guantanamo Bay media corps, estimates that the U.S. spends $454 million a year to maintain the prison facility. Among other things, the money is spent on troop salaries as well as war court costs.

Since the detention center opened its doors on Jan. 11, 2002, Gitmo has housed 779 prisoners. As of June 2014, 149 men from 22 different countries remain at the detention center.

Seventy-eight detainees, who have been cleared for release by the government, are still being held there. The American Civil Liberties Union estimates that keeping the detainees behind bars costs the country $168.1 million per year. The ACLU says it would cost the government a fraction of the cost -- $2.7 million a year – to house those same prisoners on U.S. soil.

But lawmakers like Inhofe say the detention center is the best option America has in dealing with terror suspects.

“Guantanamo Bay serves as a central and secure location for detaining and trying dangerous terrorists who are responsible for the death of Americans, and there is no cost that can be placed on keeping these terrorists off of American soil where they could influence our criminals or be targeted for a well-organized prison break,” Inhofe told FoxNews.com in a written statement.

“When it comes to the safety and security of Americans that the isolated prison provides, Guantanamo is invaluable.”