In a weekend interview, Hillary Clinton said the only way to know whether Donald Trump is "really successful" is if he reveals his tax returns.

Forbes magazine estimates Trump is worth $4.5 billion, but Trump's latest financial disclosure, filed last week, said he's worth more than $10 billion.

In 2005, New York Times reporter Tim O'Brien claimed in a book called "Trump Nation" that Trump was worth at best $250 million, when Trump and others said he was a billionaire. Trump sued the reporter for over $2 billion, but the lawsuit opened him up to a close scrutiny of his finances, reports CBS News correspondent Julianna Goldman.

When he filed the 2006 lawsuit, Trump claimed that a major part of his success was based on the "accurate perception by the financial community and public that [he] is a billionaire."

During a two-day deposition, he was repeatedly called out for exaggerating his wealth -- like when Trump told Larry King he was paid $1 million for a speech to The Learning Annex. But in the deposition, Trump conceded the actual payment was $400,000. The rest he attributed to The Learning Annex's promotional expenses.

During questioning, he acknowledged he didn't fully explain how he came up with the million dollar value.

"I don't break it down," Trump said.

The discrepancies between what Trump says his net worth is and what others estimate it to be often rests on the valuation of his brand. Over the years, Trump's international real estate developments have grown, but many of those buildings are licensed and not owned.

In 2014, Trump said his brand was worth $3.3 billion. Last year, Forbes magazine listed it at $125 million.

Behind closed doors, Trump conceded property values are often subjective.

"You said that the net worth goes up and down based upon your own feelings?" he was asked in the deposition.

"Yes, even my own feelings, as to where the world is going, and that can change rapidly from day to day," Trump responded.

In the deposition, the attorney presented estimates of his net worth by two banks where Trump had applied for lines of credit. Both concluded Trump was worth about a third of the $3.5 billion he claimed in 2005.

Trump said the numbers were wrong and the banks did not do an exhaustive search of his assets.

On the campaign trail Trump boasts that he is not a politician, but under oath, he said he spins his business like any politician.

"Have you ever lied in public... about your properties?" he was asked.

"I try and be truthful," he said. "You always - when you are making a public statement - you want to say it the most positive way possible. I'm no different from a politician running for office."

One area where Trump has broken with previous presidential candidates is refusing to release his tax returns, which would provide a fuller picture of his wealth and document his income in far greater detail than the financial disclosures he's already released.

"I think that to get a full idea of who he is, those returns are an essential piece," said former IRS commissioner Mark Everson, who served under President George W. Bush.

Everson said Trump's excuse that he's being audited doesn't pass muster.

"He doesn't have a tax reason in my opinion to fail to release those seven years that have been audited and cleared," Everson said. "I think this is a political calculation by the candidate not to release the returns."

In response to this report, Trump told CBS News in a statement: "Timothy O'Brien knows nothing about me. His book was a total failure. I haven't even heard his name in over a decade, but ultimately I had great success doing what I wanted to do -- costing this third rate reporter a lot of legal fees."

A spokesman for Trump said the original book wasn't relevant then and it's not now.

"Mr. Trump has just filed the largest personal financial disclosure form in the history of the FEC, which shows among the greatest real estate assets in the world, very low debt, tremendous cash flow and an income statement showing $557 million per year. Mr. Trump's net worth is in excess of $10 billion. The Trump brand is more valuable and popular now than ever before and Timothy O'Brien's book, which wasn't relevant, accurate or successful a decade ago, certainly isn't relevant now."

Trump's 2006 lawsuit was dismissed. He appealed and lost the appeal. The appeals judges found there were no "obvious reasons to doubt the veracity of [his] informants or the accuracy of [their] reports."

This piece has been updated with a statement from O'Brien: "Donald Trump lost his lawsuit and it didn't cost me any money to litigate it. The central tenets of his complaint -- that the book inaccurately or maliciously described his troubled business history and his exaggerations or misrepresentations around his wealth and success as a dealmaker -- didn't hold up in court. These continue to be important issues in this presidential campaign, even if Mr. Trump wants to dismiss or diminish them."