There is a short answer to the question: a lot richer than he admits.  And the longer answer is even more interesting.  Writing at Doug Ross’s Director Blue, Cliff Kincaid explains:

... [former publisher of Campaigns & Elections Magazine James] O’Brien has analyzed the financial status of Sanders and his wife, including their financial disclosure report, and has concluded they have a net worth in the range of $1.2 to $1.5 million, not the $700,000 or less that is usually reported by the media. (snip)

… his wife, Jane O’Meara Sanders, left her position as president of Burlington College under controversial circumstances and is now being accused of federal bank fraud. She left her position at the college and was given a severance package known as a “golden parachute” that also benefited Senator Sanders’ personal wealth.

Hmmm: The old “cut the wife big checks from nonprofits” gambit has a long history among Democrats.  But Jane Sanders really should have asked Michelle Obama for advice, because Jane’s $200K is chump change compared to the yearly payout Shelly got from the University of Chicago Hospital for keeping a lid on protests when it dumped nonpaying patients on other health care facilities.  Of course, there are far more opportunities for this sort of enrichment in Chicago than in Burlington.  And face it: Burlington College was in tough financial shape at the time, with nothing like the resources of the U of C.

... as noted by Bruce Parker, a Vermont reporter for Watchdog.org—Senator Sanders should be asked to explain how his opposition to severance packages for corporation executives squares with his wife getting a cushy severance of $200,000.

Then there is a little smoke on Ms. Sanders’s banking:

Brady C. Toensing, a partner with the law firm of diGenova & Toensing, has filed a legal complaint with federal authorities requesting an investigation into apparent federal bank fraud committed by Ms. Sanders. His complaint was sent to Eric S. Miller, the U.S. Attorney for the District of Vermont, and Fred W. Gibson, Jr., Acting Inspector General with the Federal Deposit Insurance Corporation.

Sanders is hiding some of his assets (legally):

O’Brien says that Sanders’ financial disclosure forms are incomplete. “For someone who doesn’t care about money, he goes a long way to cover up his true net worth,” he says. “Bernie does not disclose the value of real estate holdings. He can. He is not required to, but he could if he chose. It is known that he and/or his wife own at least two homes—one with rental income in Vermont and one near Capitol Hill where the median home value is $722,000.”

I have nothing against people buying houses and accumulating a net worth.  But if Sanders’s wife executed a hypocritical severance package of the sort Bernie denounces, that is outright hypocrisy.  And if he and his wife have accumulated a net worth that makes them millionaires, it sounds bad to his student loan-indebted followers.  A million bucks ain’t what it used to be, but Sanders supporters don’t realize that.