The Wall Street Journal has a great piece today on this question, which is at the heart of a lawsuit between Tim O'Brien, a New York Times editor and author of a distinctly unauthorized Trump biography, and "the Donald" himself. According to the Journal:
The world famous real-estate developer and television personality has consistently said it's in the billions. A 2005 book citing anonymous sources said it was between $150 million and $250 million. Mr. Trump sued the writer for defamation. He alleged damage to his reputation that caused him to lose out on future deals in locales from Philadelphia to Kiev.
But, alas, it sounds like the rigor of Trump's accounting techniques leaves something to be desired:
In the deposition [resulting from the lawsuit], Mr. Trump said that his 2007 estimate of his net worth -- over $4 billion -- is "a very conservative number, in my opinion." He also said $6 billion is a good number, counting his brand value. ...
[I]n a November 2007 Wall Street Journal interview cited by Mr. Ceresney [O'Brien's lawyer], Mr. Trump said he had sold out units at an eponymous condo-hotel project in Hawaii. "The building is largely owned by me," he said in the interview. But in the deposition, Mr. Ceresney produced the licensing agreement for the project. Mr. Trump wasn't a major equity holder in the project, it showed, a fact Mr. Trump didn't dispute.
"Because this is such a strong licensing agreement that I consider it to be a form of ownership," Mr. Trump said. "I'd rather have this than own the building," he said. Moments later he said: "I would say that it could be interpreted to be a form of ownership in the building."
The impression you get reading the piece is that Trump isn't so much a preposterously rich guy as a kinda wealthy guy who makes money impersonating a preposterously rich guy. Which is clearly why the book is potentially so damning. If you made money on the lecture circuit as, say, a decorated general, and a credible source alleged you'd only risen to the rank of lieutenant, you'd probably view the allegation as a real threat to your livelihood.
Relatedly, I'm not a lawyer, but my sense is this would weaken a defamation case, not strengthen it:
In the deposition, given to lawyers representing the book's author, Timothy O'Brien, and its publisher, a unit of French-based Lagardere SCA, Mr. Trump described his public persona. "I'm not different from a politician running for office," he said.
Isn't the legal standard for defaming a politician incredibly high? It certainly doesn't seem like accusing one of having less money than they claimed would cut it. (Maybe impugning their chief claim to fame would get you closer, but it still seems implausible. People write stories all the time about how so-and-so politician claimed to have single-handedly reformed welfare or cut crime in half, and, so far as I know, they don't lose defamation suits.)