(CNN) Bernie Sanders, the clear front-runner for the Democratic presidential nomination, was asked over the weekend how much his various plans would cost if implemented. He didn't know.

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Cooper: Do you know how all -- how much though? I mean, do you have a price tag for -- for all of this?

Sanders: We do. I mean, you know, and -- and-- the price tag is -- it will be substantially less than letting the current system go. I think it's about $30 trillion.

Cooper: That's just for "Medicare for All," you're talking about?

Sanders: That's just "Medicare for All," yes.

Cooper: Do you have -- a price tag for all of these things?

Sanders: No, I don't. We try to -- no, you mentioned making public colleges and universities tuition free and canceling all student debt, that's correct. That's what I want to do. We pay for that through a modest tax on Wall Street speculation.

Cooper: But you say you don't know what the total price is, but you know how it's gonna be paid for. How do you know it's gonna be paid for if you don't know how much the price is?

Sanders: Well, I can't -- you know, I can't rattle off to you every nickel and every dime. But we have accounted for -- you -- you talked about "Medicare for All." We have options out there that will pay for it.

What? So, Sanders not only a) isn't sure how much all of his proposals would cost but also b) isn't able to say how he would pay for these programs. That strikes me as a potential weak spot if/when Sanders winds up as the Democratic nominee against President Donald Trump.

Which is the point that former Vice President Joe Biden's campaign immediately sought to make. "For the second time in the last month, Senator Sanders has admitted that he does not know the astronomical price tag that his massive new programs would force onto American families," said Biden deputy campaign manager Kate Bedingfield. "That's untenable."

And in a weekend memo from the Democratic centrist group Third Way warning the party of the perils of nominating Sanders, authors Jonathan Cowan and Matt Bennett write:

"Experts estimate that Sanders' major proposals would cost a staggering $60 trillion and would double the size of the government (while his tax plans fall $27 trillion short of paying for it). There's a reason that, when pressed on the cost of his plans, Sanders simply refuses to answer, saying he actually has no idea and 'no one does.'"

That $60 trillion number comes from The Atlantic's Ron Brownstein, a CNN contributor, who broke down the costs of Sanders' proposals like "Medicare for All," the "Green New Deal" and free tuition at public colleges and arrived at that stunning price tag.

Just how big a number is that? This, from Brownstein , puts the $60 trillion in spending proposals in very clear context:

"The Vermont independent's agenda represents an expansion of government's cost and size unprecedented since World War II, according to estimates from his own website and projections by a wide variety of fiscal experts.

"Sanders' plan, though all of its costs cannot be precisely quantified, would increase government spending as a share of the economy far more than the New Deal under President Franklin Roosevelt, the Great Society under Lyndon Johnson or the agenda proposed by any recent Democratic presidential nominee, including liberal George McGovern in 1972, according to a historical analysis shared with CNN by Larry Summers, the former chief White House economic adviser for Barack Obama and treasury secretary for Bill Clinton."

Now consider that there is no estimate from any credible budgeting service that suggests that the government would be able to bring in the sort of revenue needed to pay for that spending surge over the next decade. Sanders' plan to raise taxes on the wealthy and corporations would close some of that gap, but a study from a fellow at the Manhattan Institute (a conservative think tank) cited by Brownstein suggests the top end of revenue from the Sanders' tax increases is $23 trillion.

Sanders doesn't talk much about the price tag of what he's proposing or the very real likelihood that his tax plan will not be enough to fill the spending gap he would create. Which makes sense -- because, politically speaking, the idea of raising taxes on what we broadly consider the middle class isn't terribly popular among, well, the middle class. (Raising taxes on the wealthy or corporations, on the other hand, is a stone-cold winner politically.)

But Sanders once did admit the harsh reality of how his plans would be paid for during a debate over the summer. Here's the exchange between Sanders and NBC's Savannah Guthrie (bolding is mine):

Guthrie: Will you raise taxes for the middle class in a Sanders administration?

Sanders: People who have health care under "Medicare for All" will have no premiums, no deductibles, no copayments, no out of pocket expenses. Yes, they will pay more in taxes, but less in health care for what they get.

So, here's what we now know about Sanders' plans for America:

1) He isn't sure how much they will cost.

2) He isn't totally sure how he will pay for them.

3) It's likely they will be paid for by an increase in taxes on the middle class.

Whoo boy. Maybe Sanders is right that America is ready for a fundamental reorienting of how we value ourselves, our society and our money. But if he's not -- and this election winds up being like virtually every other election in which people vote on who is going to let them keep more of their money -- then Sanders (and Democrats by extension) have a big problem.