NEW YORK (Fortune) -- Is this the biggest overnight price increase you've ever heard of - 359%? On April 10th, which is tomorrow, that will be the leap in "transaction fees" paid by investors to the U.S. government.

This might not be the worst news you ever heard, so hold your ire until we explain. In a fact little-known to investors, market-makers like the New York Stock Exchange and NASDAQ must pay fees to the government that are based on the dollar volume of their trading in stocks. These establishments, however, don't really bear the freight. Nor, in most cases, do their broker-dealer members.

Instead, the brokers pass the cost along to their customers - that is, investors - who pay it when they sell stocks. (Were the fees also to be charged on purchases, double-counting would occur; so the fees only apply to sales). Check out the confirmation you get on each sale to see the fee.

At the NYSE level, to hit the upper link of this chain, the fee for one million dollars of trading today, April 9th is $5.60, and tomorrow it will more than quadruple, leaping to $25.70. And why? Because in 2002 Congress passed a law saying it should.

The history here goes back to the tech bubble of a decade ago, when the dollar volume of trading vaulted to then-unprecedented heights. In the system then in place, the transaction fees rose proportionately, hitting $33.33 per million dollars in the government's fiscal 2000 and looking like they might go to the sky.

Congress then got rid of proportionality by passing what is known as the Fee Relief Act. Through 2011, this law said, the Securities & Exchange Commission, which collects the fees for the Treasury, should both aim for an annual total specified by the law and adjust rates at least once a year so as to get close to the target.

As is the case with everything about the government, there are additional complications, the main one being that a fee change can't go into effect if the SEC's budget hasn't been approved. (Do not be misled by that fact: The SEC's budget, $960 million for the fiscal year to end September 30th, is not dependent on the transaction fees).

For the current fiscal year, the SEC is supposed to gather $1.023 billion in transaction fees. But in January, looking at prospects for the first six months of the year, the Commission estimated it would take in only $190 million for that period - not even 20% of the amount needed!

This shortfall is occurring because certain events conspired: Approval of the SEC's budget was delayed; the very low fee of the previous year, that aforementioned $5.60, was consequently held over; and stock prices cratered, which meant the fee was applied to relatively low dollar volumes.

The Fee Relief Act - ill-named for this moment - thereby kicked in, requiring the SEC to engage in certain algorithmic calculations to predict the rate it will need from April through September to hit the $1.023 billion target. The math said, unsurprisingly, that a huge increase was called for - and the law made it mandatory. And that's why the rate happens to be rising tomorrow, April 10th, by 359%. (Calendar note: Said day is, of course, Good Friday, when the NYSE is closed; that makes Monday, the 13th, the day that counts).

So let's get to the nub of it all. Even if you're an active investor, don't tell your kids they can no longer plan on college because of this sudden and oppressive price increase. The fee for even a $100,000 trade - say, a sale of 2000 shares of a $50 stock - will rise to only about $2.50. You might be able to stand that.