Filling up hasn't been this cheap in a long time. (AP/Rich Pedroncelli)

There are already signs that gasoline prices, which have fallen to their lowest level in the past six years, are making it easier for Americans to get by. The sudden, 20 percent decline in gas prices has saved the average person $520 since June, and people are putting that extra money to use. Wal-Mart said last week that its quarterly sales had increased for the first time in nearly two years, and other retailers are seeing better business, too.

Yes, cheap gas is good for the economy, but in the same way that all those empty seats at FedEx Field are good for Redskins fans: the fact that it's easier to get a ticket isn't much consolation for a 3-7 record.

Likewise, falling gas prices aren't exactly cause for celebration. It is true that they're partly a result of fracking, which has made fuel more readily available in the United States, and the response of Saudi Arabia and the other oil-exporting states, who are nervous enough about American drilling that they're not jacking up prices, as they might have done in the past. These are good things (serious environmental worries aside), and they've created a worldwide glut of oil.

What's worrisome is that no one is buying all that oil.

The unfortunate truth is that low prices for gasoline are a symptom of a sickly global economy. It isn't just gas prices that are falling as consumers make do with less and businesses put off new investments in Europe, China and Japan. Metals such as copper, silver and platinum are cheap, too, and so are crops like corn, soybeans and wheat.

If prices continue to fall, the economy could be in real danger, because there won't be much central banks can do to encourage spending with interest rates already at or near zero in most of the world. Lower prices might be good in themselves, but it's what they reveal about the way things are headed that's cause for concern.

Welcome to Wonkbook. To subscribe by e-mail, click here. Send comments, criticism or ideas to Wonkbook at Washpost dot com. Follow Wonkblog on Twitter and Facebook.

What's in Wonkbook: 1) Obamacare enrollment opens 2) Opinions: The working class, net neutrality, Ebola and Mark Udall 3) Government shutdown threatened 4) Ferguson tapes released 5) Guns lead to crime, corporate tax reform, the chocolate deficit, and more

Chart of the day:

Tuition and fees

Is the cost of college reaching a plateau? Tuition and fees increased only 1 percent above inflation last year. Danielle Douglas-Gabriel in The Washington Post.

Number of the day: 311. That's the speed, in miles per hour, of Japan's newest passenger train. BBC.

1. Top story: Health care enrollment reopens, quietly

No news is good news for Obamacare. "Many people were able to create accounts in minutes instead of the hours or weeks it took during the first sign-up season." Stephanie Armour, Louise Radnofsky and Anna Wilde Mathews in The Wall Street Journal.

The federal exchange received 100,000 applications over the weekend. The figure given by the administration was in sharp contrast to the 248 people who were able to apply online on the first two days of enrollment last year. Some problems related to consumers being unable to access their accounts. Robert Pear in The New York Times.

But premiums are increasing. New plans tend to be costlier than older plans, and the average increase for existing plans is 7 percent -- although prices vary widely. At the same time, insurance companies are offering consumers more options than they did last year. Dan Keating and Jason Millman in The Washington Post.

KLEIN: Meanwhile, everyone is focusing on Jonathan Gruber. Republicans have seized on the economist's remarks that the White House designed Obamacare to confuse people, which is ironic because through his research and speeches, Gruber did more than anyone to clarify how the law would actually work. Vox.

2. Top opinions: The working class, net neutrality, Ebola, Mark Udall and the NSA

LEONHARDT: Working-class voters still support Democrats, but that could change. Democrats won easily among voters making less than $50,000 a year in the midterms, as they have for decades. A payroll tax cut would be one among several ways for Republicans to make their case to the working class. The New York Times.

RATTNER: Incomes fell for 90 percent of Americans over the past four years. Incomes for the richest 10 percent improved slightly, which raises the question of why the United States doesn't do as much as other developed countries to redistribute wealth. The New York Times.

CROVITZ: Regulating the Internet like a utility would be dangerous. Putting Internet providers under Title II could have consequences that Obama isn't prepared for. The technology sector would have to ask for federal permission before introducing all kinds of new ideas. The Wall Street Journal.

KRUGMAN: Government can and does work. The partisan panic over Ebola was completely unjustified, it turns out -- just another example of public officials instituting imperfect but nonetheless effective policies that succeed despite politically motivated criticism. The New York Times.

FRIEDERSDORF: Mark Udall should reveal "the abuses the government is trying to hide." It's easy for intelligence officials to violate the law and the Constitution when their activities are secret. The defeated senator, before he leaves office, can make classified information public in his capacity as a legislator without fear of prosecution. The Atlantic.

Hip-hop interlude: "Close Your Eyes" by Run the Jewels.

3. The government shutdown clock begins

Conservative Republicans want to respond to a planned executive order on immigration with a short-term spending bill. Obama's plan to order a delay in deportations for millions of undocumented immigrants has led to a dispute between the G.O.P. leadership and conservative members like Sen. Jeff Sessions, the chairman of the Budget Committee. Sessions and others want to be able to make a credible threat to shut down the government, something Mitch McConnell has promised not to do. Paul Kane in The Washington Post.

One strategy would be to pass a series of short-term bills, setting up a crisis every several months. With a series of spending bills, Republicans would have plenty of chances to force concessions from the White House. McConnell's preferred strategy would be to return the Senate to regular order, which would mean passing an appropriations bill to fund the government through the end of the year. Robert Costa in The Washington Post.

Congress must pass a new spending bill, one way or another, by Dec. 11. Obama could wait until after that date to issue an executive order on immigration. Molly Hooper in The Hill.

BALZ: The White House is tired of waiting for Republicans. After six years, Obama administration officials have more or less given up hope that Republicans will cooperate with them on a major policy issue. They feel that if they wait for Congress to act on immigration, they'll be waiting forever. The Washington Post

DOUTHAT: An executive order would go beyond Obama's authority. "No liberal has persuasively explained how, after spending the last Republican administration complaining about presidential 'signing statements,' it makes sense for the left to begin applying Cheneyite theories of executive power on domestic policy debates." The New York Times.

4. Tapes of Ferguson, Mo. shooting released

Dispatch tapes show that the encounter that ended in the shooting was very brief. About a minute lapsed between when Officer Darren Wilson radioed to say that he was stopping Michael Brown and a friend and when he fatally shot Brown. The tapes also show that Wilson knew that other officers were looking for two people who had just stolen merchandise from a store. But that's about all they demonstrate conclusively. Robert Patrick in the St. Louis Post-Dispatch.

Surveillance footage shows Wilson after the shooting. "The videos — grainy and in black and white – are the first confirmed images of Wilson from the night of the shooting... Wilson’s supporters have said he was seriously injured in the altercation with Brown. The video released this weekend shows Wilson moving normally but does not provide a close view of his face." Wesley Lowery in The Washington Post.

Organizers in Ferguson are looking for ways to prevent violence. As they stock up on items such as gloves, maps, bandages, and goggles, organizers are trying to agree on how far protests should go if Wilson is not indicted. "Beyond their carefully orchestrated plans for a series of shows of protest and civil disobedience, leaders here acknowledge that there are disagreements about what form of response is fitting and whether militant acts might spill over into violence." John Eligon, Julie Bosman and Monica Davey in The New York Times

What about the White House's review of police militarization? Obama administration officials had pledged to review a number of federal programs that help provide local police with heavy weapons, including one that distributes surplus military equipment. But that review has not produced any public results so far. Evan McMorris-Santoro at BuzzFeed

Koala diplomacy interlude: Republicans may have taken the Senate, but Barack Obama got to hug a koala. Ezra Klein for Vox.

5. In case you missed it

More guns, more crime. New research debunks a crucial talking point of the gun-rights movement. Christopher Ingraham in The Washington Post.

Federal law enforcement is relying more on undercover agents. Even the Agriculture Department has 100 plainclothes officers now. "Some agency officials say such operations give them a powerful new tool to gather evidence in ways that standard law enforcement methods do not offer, leading to more prosecutions. But the broadened scope of undercover work, which can target specific individuals or categories of possible suspects, also raises concerns about civil liberties abuses and entrapment of unwitting targets." Eric Lichtblau and William Arkin in The New York Times.

Some Republicans are ready to move ahead with corporate tax reform. The two main points of contention with the White House seem to be whether to address the corporate and individual tax codes separately or in one piece of legislation, and whether corporate tax reform will also raise revenue for investment in infrastructure. Lori Montgomery in The Washington Post.

Republicans use secret Twitter accounts to dodge election laws. Election regulations prohibit coordination between campaigns and outside groups, but the campaigns made the information "public" using Twitter. Chris Moody for CNN.

The State Department's email system was shut down worldwide on Friday for security. Technicians were working to make improvements and repair damage after a suspected attack on the unclassified system last month. Matthew Lee for the Associated Press.

Protesters are hounding the Federal Reserve. They want the public to have a chance to weigh in when new presidents are chosen, to be sure that the central bank listens to people on the margins of the economy. Danny Vinik in The New Republic.

We're running out of chocolate. The world is consuming more cocoa than it produces, and the deficit is expected to widen due to increasing demand, disease and bad weather. Roberto Ferdman in The Washington Post.