Most billionaires lie about their net worth. Donald Trump just happens to lie more than most.

What's a couple billion dollars worth to Donald Trump? It turns out it might be more than he claims: According to Forbes editor Randall Lane, it's not unusual to catch billionaires lying about their net worth — and Trump has a long history of exaggerating his numbers.

"There's been a rule at Forbes for the last 30 years: Take what Trump says and divide by three," Lane said Tuesday on MSNBC's Morning Joe.

This year Forbes spent more time with Trump's file than anyone else's in the history of the Forbes 400, which lists the nation's top billionaires. Forbes' final number ultimately put Trump's worth at $4.5 billion — less than half of the $10 billion he's claimed to have in the past, and less than the worth of the average billionaire on Forbes' list ($5.8 billion).

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But don't blame Trump for wanting to obscure his true worth: It turns out that most billionaires on the list do the same thing. "There's a lot of lies," Lane explained. "Half lie higher and half lie lower."

Perhaps most curious of all is that Trump's personal brand has actually grown significantly since he started his run for the presidency. "Every brand expert we talked to said, 'You know what? The summer of Trump has made his brand actually more valuable,'" Lane said, dismissing the idea that the billionaire's controversial comments cost him money or fans. "And yes, it's not as broad, but it's deep. Those who like him really like him." Experts say the "Trump" brand is worth anywhere between $125 million and $1.1 billion.

Hear more from Lane below. Jeva Lange