PALM BEACH, Fla. — Before calling the White House home, Donald Trump used to wonder why any president would ever leave.

"If you're in the White House, who wants to take a vacation?" he once asked a campaign crowd in Spencer, Iowa. Noting the limited time a president has there, he criticized President Obama for vacationing "when there's so much work to be done."

"You're in the White House," he said. "What's better than the White House? Why these vacations?"

But since his January inauguration, President Trump has spent seven of 13 weekends at his Palm Beach, Florida estate. According to NBC News’ estimates by Sunday Trump will have spent 28 percent of his term traveling to or staying at Mar-a-Lago. Trump hosted his second world leader at the "winter White House" last weekend, with Chinese President Xi Jinping joining Trump for meetings Thursday and Friday. Trump stayed the rest of the weekend, frequenting his nearby golf club on both days.

It's not just a question of travel time, but of ethics and cost efficiency, according to watchdog groups and ethics experts.

While presidents have always traveled on the taxpayer's dollar — the Obamas were partial to Hawaii and Martha’s Vineyard while President George W. Bush frequented his Crawford, Texas ranch — Trump’s travel is "unprecedented," one expert says, because he's repeatedly visiting his own privately owned commercial property at Mar-a-Lago.

"It's just another example of his consistent efforts to exploit public office for private gain," ethics expert Steve Schooner told NBC News. "He's using his official office and the fact that people have to travel with him, meet him, and follow him to promote his commercial enterprise, in this case his privately owned club."

"I can’t think of anything like this that we’ve seen at anytime in the modern era," the George Washington Law School professor told NBC News. The Mar-a-Lago property is still owned by Trump, who placed his holdings in a trust overseen by his eldest son, Donald Trump Jr., and Trump Organization CFO Allen Weisselberg before assuming office, prompting Schooner to lament that the president "should have divested from his properties to begin with."

Since Trump became president the cost of membership at Mar-a-Lago has doubled, with guests now paying $200,000 just to join.

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"This is a privately owned club that for all intents and purposes was just another golf property in Florida before, that almost now is something that Americans immediately recognize," Schooner continued. "Imagine what you would have to pay to get that kind of brand recognition. That's extraordinary."

President Donald Trump and First Lady Melania Trump greet a marching band as they arrive at Trump International Golf club to watch Super Bowl LI between the New England Patriots and Atlanta Falcons in West Palm Beach, Florida on Feb. 5, 2017. Carlos Barria / Reuters

Aside from concerns over who is profiting from Trump's trips, is the question of who's paying for them.

Over two-dozen A-listers from Trump’s administration have visited the property over the past three months, along with high-level job applicants, and even New England Patriots owner Robert Kraft, among others. While passing the days in Palm Beach, Trump has been known to spend time at his nearby golf clubs, where he may or may not take the time to hit a few balls amidst meetings and phone calls.

A White House official told NBC News that when President Trump travels to Mar-a-Lago "he is staying at his home, the Southern White House, and has minimal staff with him." Hotels and meals for those federal employees involved in the trips are covered by the government, according to the official.

Even though Trump is, for the most part, working weekends, conservative watchdog group Judicial Watch thinks there are less costly ways to do it.

RELATED: Government Accountability Office to Examine Cost, Security of Trump Florida Trips

They estimate the cost of a back-and-forth trip from Washington to Palm Beach at about $1 million, with Air Force One costing $142,380 per hour, according to information Judicial Watch says it obtained through a Freedom of Information Act request. Other estimates put each trip's price tag as high as $3 million.

White House Press Secretary Sean Spicer has defended the travel, saying Mar-a-Lago "is where [Trump] goes to see his family. He brings people down there. This is part of being president."

Trump is not the only president to rack up a high tab while traveling, however. Though the White House wouldn't say exactly how much Trump's trips cost him, estimates by the Government Accountability Office show that a weekend trip taken by President Obama in 2013 to Chicago and South Florida cost about $3.6 million.

Still, Judicial Watch President Tom Fitton says the cost to taxpayers in either case is too high, calling for Trump "to try to figure out ways to save money for the taxpayer" and applauding him for using his Virginia golf clubs when he’s looking to get away. Fitton doesn’t question that Trump’s working — "whenever a president travels, he’s doing work," he says — but he does "question whether it’s necessary to travel to a particular place to do the work."

Fitton issued similar calls for cost awareness and transparency during the Obama years, noting now "a little bit of anti-Trump animus" in the media’s criticism of Trump’s presidential travel.

The use of Mar-a-Lago to host foreign leaders, however, is justifiable in his eyes.

"It’s the president’s home," Fitton told NBC News. "It’s probably a nice way to visit with leaders and get to know them and to do some important business with them. So it’s not the end of the world that he hosts the president of China, or anyone else, down there."

The Mar-a-Lago Resort in West Palm Beach, Florida on Feb. 11, 2017. Joe Raedle / Getty Images, file

In addition to hosting President Xi, who did not stay at Mar-a-Lago during last weekend's bilateral summit, Trump personally gifted Japanese Prime Minister Shinzo Abe a weekend-long stay at his hotel during their tete a tete several weeks ago.

The cost of Trump’s summit with President Xi hit hard on a local level, with the Palm Beach County community footing the bill, an estimated $1.5 million for Thursday and Friday’s U.S.-China summit.

Palm Beach County Sheriff Ric Bradshaw told NBC that each day President Trump spends in Mar-a-Lago costs his department $60,000.

County Commissioner Dave Kerner noted Bradshaw's cost evaluations, making the point that with Trump governing from Palm Beach County, "it becomes a very significant financial burden on the taxpayers" here.

"While he may treat it like it’s the Southern White House, it’s not," Kerner said, worrying about skyrocketing costs that the county hadn’t budgeted for. Kerner hopes for reimbursement from the federal government to offset the burgeoning bill of Trump’s visits to the area and has even suggested turning Mar-a-Lago into a special taxing district that would impose a levy on the resort to pay the president's security costs, according to an Associated Press report.

Democrats have also seized on Trump’s travel, using it as a chance to call for increased transparency of who’s visiting with Trump at Mar-a-Lago, as well as asking the Government Accountability Office to look into taxpayer costs and security measures related to Trump’s use of the property.

In the eight years of his presidency, Barack Obama’s total travel costs were at least $97 million. At the current rate, the costs of President Trump’s travel could well exceed that.