Donald Trump likes to boast that he's funding his own presidential campaign and thus is not beholden to the special interests who own the "rigged" political system.

He also could be making a nice profit while he's at it.

Throughout the past year, "a large share of Trump's campaign money has been spent paying himself for the use of his Boeing 757, his smaller jet, his helicopter, his Trump Tower office space, and other services supplied by Trump businesses," Pulitzer Prize-winning journalist David Cay Johnston writes in his new book, "The Making of Donald Trump." In July alone, the Trump campaign spent some $2 million on private jets, plus another $500,000 for his 757, which includes gold-plated seatbelt buckles. Johnston adds:

By law, Trump must pay charter rates for his aircraft and market prices for services from his other businesses. This anticorruption law was designed to prevent vendors from underpricing services to win political favors - a legacy of a time when no one imagined that a man of Trump's presumed immense wealth would buy campaign services from himself. In 2016, the law ensures that Trump makes a profit from his campaign.

None of this is likely to surprise Trump's supporters. As the candidate himself is fond of saying, he's a really smart guy.

He also apparently needs the money. Trump's various companies are carrying at least $650 million in debt -- "twice the amount than can be gleaned from public filings he has made as part of his bid for the White House," the New York Times reported over the weekend.

This might help explain why Trump continues to run his campaign in such an unusual way, even though since becoming the GOP's standard-bearer he's partly abandoned self-funding and is now raking in an impressive amount in contributions. The Associated Press reported Sunday that Trump's campaign expenses "more than doubled last month, even as the Republican presidential nominee held his payroll to about 70 employees, aired no television advertisements and undertook no significant operational buildout across the country."

So where's the money going?

Almost half of the campaign's $18.5 million in July spending went to an internet marketing firm with no political-campaign experience but is a "crossover vendor from Trump's real-estate organization." The president of the firm, Brad Pascale, serves as the campaign's digital-marketing director. The campaign's "big expense," writes AP, was online fundraising. Trump is spending money to raise money so he can pay himself for campaign services.

While Trump is keeping his outlay for actual campaigning low, Hillary Clinton's campaign, in contrast, has about 700 employees and has already spent millions of dollars on TV ads, primarily in so-called "battleground states." The Democratic presidential nominee's spending and electioneering approach fall in line with professionals' expectations for a 21st-century major-party candidacy. In July, her campaign raised $52 million, about $15 million more than the Trump effort brought in.

Trump's unconventional campaign has led to speculation that he's pursuing the presidency for business reasons -- to "build his brand," leverage relationships with debt-holders and make money.

"The success of his empire depends on an ability to get credit, to get loans extended to his business entities," Richard W. Painter, University of Minnesota law professor and former White House chief ethics lawyer during the George W. Bush administration, told the New York Times. "And we simply don't know a lot about his financial dealings, here or around the world."

The Trump campaign, which just put out its first general-election TV ad, dismisses such talk. Trump, who has refused to release his tax returns, says he's running for president for one reason: to save struggling Americans from a corrupt economic system, declaring at the Republican National Convention last month, "I am your voice."

Johnston, for one, isn't buying it. The reporter and author told the liberal-advocacy news site AlterNet:

"Listen, for Donald's entire life he has broken the rules or ignored the rules and it's done well for him. So why would [he] behave any other way?"

-- Douglas Perry