For the economically challenged news industry, the calculation looks something like this: Seven days of coverage spread over two weeks. Thousands of reporters scrambling for a limited pool of stories. Enormous expenses. Limited viewer, reader and advertising upside.

Whatever their value as news events, the political conventions — which ended with President Obama’s acceptance speech Thursday night — are the single most expensive events for the American media to cover. Every four years, news organizations commit millions of dollars to the care and feeding of the journalistic army — 15,000 strong this time around — that encamps in the two host cities.

These investments of money and manpower come at a time when even the political parties have been scrutinizing the length and cost of their quadrennial meetings. Republican leaders last week openly debated whether their four-day, $100 million affair (shortened by one day due to Hurricane Isaac) had become excessive, that three days might be plenty.

They are unlikely to get much argument about that from the news media. “It’s not my place to tell anyone how to run a convention or how long to run it, but it wouldn’t bother me at all if they tightened it up,” said John F. Harris, editor in chief of Politico, which sent 70 journalists to Tampa and Charlotte. “The timing is unquestionably a drag. They’re bunched too close together. And it’s weird having one after Labor Day.”

But, he adds, “We’ll be there as long as there are conventions, I guess, and pretty enthusiastically at that.”

Department of Health and Human Services Secretary Kathleen Sebelius talks with the press at the convention center in Charlotte before the opening of the DNC. (Lucian Perkins/For The Washington Post)

The broadcast networks have been cutting back their coverage of the conventions since the gavel-to-gavel days of the “I Love Lucy” era. The legacy networks — ABC, NBC and CBS — broadcast 90 minutes per night during the 1996 conventions; now they set aside just an hour (between 10 and 11 p.m.), devoting the rest of their prime-time hours to entertainment shows.

The cutbacks have been propelled by the rise of cable TV news, video streaming online and the decline and aging of convention audiences — roughly two-thirds of viewers are older than 55. TV ratings for the conventions this year have been down by 20 percent or more compared with 2008, although they’ve been near those of the 2004 meetings. Nevertheless, the audience watching on cable and broadcast TV has been getting older and smaller for more than 30 years.

Even at an hour per night, the networks lose money on the conventions. The reason: Lengthy speeches — Bill Clinton’s clocked in at 48 minutes on Wednesday — provide few opportunities for commercial breaks.

“When was the last time you saw a scripted or reality show or sports show even or a newsmag [go as long] without an ad break?” said one network executive who asked not to be named because he’s not an official spokesman. “So you’re losing money on the hour each night, given the costs” of coverage.

The networks can’t quit altogether, however, because “you can’t be a credible news organization if you don’t cover” the conventions, he said.

For cable news, with its 24-hour cycle, the calculation is somewhat different. Round-the-clock coverage means lots of commercials. Still, there’s “absolutely no way to determine” whether CNN covered its costs during the conventions, said Sam Feist, the network’s political director. Political reporting “is an all-year affair for us and the conventions are an important part of that. They’re important to our brand as the place you can turn to for political news that’s based on reporting, not opinion.”

Beyond the housing, feeding and transporting of journalists, there’s a lengthy list of additional expenses for the news media, although no one contacted for this article would divulge specifics. While news organizations don’t pay for convention work spaces (those are assigned by the Senate Daily Press Gallery in consultation with the Republican and Democratic national committees), companies do pay for equipping and outfitting their space. That means hooking them up with electricity, phones and Internet connections and providing security, food service, cleaning and office equipment, usually at non-negotiable rates set by convention-approved contractors. TV networks pay extra for the skyboxes that overlook the convention center floor.

The media are also dependent on the political parties for hotel assignments (the parties typically set aside the better hotels for their donors and delegates). You can count on a convention markup, too: Reporters from The Post, New York Times, and Bloomberg, for example, stayed in a Tampa hotel last week that charged $179 a night for a room. After the circus left town, the same room went for $71 this week.

Editors and news executives acknowledge that the conventions aren’t as dramatic as those of yore — no floor fights, no nominating suspense — but say they’re still newsworthy. As tribal gatherings, they are a natural peg for stories about issues, candidates and campaign strategies. Plus, conventions are valuable for source-building and networking, they say.

“I had the opportunity in Tampa and Charlotte to reconnect with sources of mine from decades ago,” said New York Times executive editor Jill Abramson. “I picked up new insights and story ideas that will be of value in our 2012 coverage.”

Abramson says the Times — one of many newspapers affected by a long and deep advertising recession — sent fewer reporters to the conventions this year. Although she declined to provide specifics, Abramson said the Times’s overall head count did not fall, given that it has replaced some reporters with videographers, producers and technicians for its live video reports.

Similarly, along with its contingent of reporters, editors, bloggers, columnists and photographers, The Washington Post shipped “a caravan of equipment” to the convention sites to equip its remote TV studio and outfit its live discussion panels, said executive editor Marcus Brauchli.

Huffington Post sent 18 reporters, a videography crew and eight “citizen journalists.”

Some media outlets went further — much further. Bloomberg, the multimedia news outfit, rented 16,500 square feet in a Gold’s Gym adjacent to the convention center in Charlotte (and 15,000 square feet in Tampa), and transformed it into a combination work space, panel-discussion setting, TV studio and reception center. The company had 90 journalists on site at both conventions, one of the most of any news organization. It offset its costs by bringing in sponsors such as AT&T and America’s Natural Gas Alliance.

“I think it’s been an invaluable investment” both for Bloomberg’s journalism and its brand name, said Al Hunt, Bloomberg’s Washington editor, who has covered conventions for 40 years. “We’ve expanded our presence in Washington enormously in the last year. I think this gives us a chance to show off.”

Lisa de Moraes contributed to this report.