Mark Cuban questions if Donald Trump is actually a billionaire

Donald Trump is pretty adamant that he is "really rich," claiming a net worth of more than $10 billion. Some have questioned that valuation during his presidential campaign -- and you can count internet mogul Mark Cuban among those skeptics.

Appearing on 77 WABC's "Bernie and Sid," the Dallas Mavericks owner said the presumptive GOP nominee's lack of transparency makes it difficult to determine his actual net worth.

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Trump has repeatedly refused to release his tax returns, which it is customary for all presidential candidates to do, citing an ongoing IRS audit. He did file a financial-disclosure form with the Federal Election Commission, which he called "the largest in the history of the FEC."

Cuban told the radio hosts he had looked at the filing and wasn't convinced by Trump's self-proclaimed net worth. He did give Trump credit for being good at real estate, but said he "wasn't very good at brands for non-real estate products," calling some of his deals "desperate."

See some of Cuban's most thoughtful business quotes:

"So when you're putting your name on steaks, and you're putting your name on water, you're putting your name on playing cards, you're putting your name on all this nonsense, right? You're not gonna make big bucks, no matter what. It's not like Trump Steaks were gonna make him $100 million. It's not like it was gonna make him $5 million."

Trump is notably sensitive about suggestions he's not worth quite as much as he says he is. In 2007 he sued an author who suggested he was worth between $150-250 million.

Cuban has criticized Trump before, telling Bloomberg in February he thought he would make a better president than the real-estate icon. More recently he suggested he would be open to being the running mate for either Trump or rival Hillary Clinton.

In the most recent Forbes Rich List, Cuban's net worth was estimated at $3.2 billion, which would make him the 527th richest person in the world.