NEW YORK (CNN/Money) - The summer concert season is looking a bit sickly these days, and not just because Britney Spears has a bum knee.

At least four top draws -- Spears, Christina Aguilera, Marc Anthony and Jessica Simpson -- have cancelled some or all of their tours, costing themselves, promoters and T-shirt vendors, not to mention people like stadium security guards, millions of dollars in lost ticket and merchandise sales, publicity, and income.

But these bowouts tell only half the story.

The other half: ticket sales have slowed across the $2.5 billion concert industry.

In response, Clear Channel Entertainment, the world's largest producer of live entertainment, has slashed ticket prices twice in recent weeks.

In two markets, San Antonio and San Francisco, the company has recently offered up lawn tickets to all of its shows for just $10 and $20 apiece.

"It's almost like the business went off a cliff and it has not come back yet," said Gary Bongiovanni, the editor-in-chief of Pollstar, a weekly magazine that tracks the concert industry.

According to Michael Issac, a Las Vegas broker and president of PreferredTicket.com, brokers who bet heavily on huge demand are now unloading tickets at below face value. Madonna tickets for her "Reinvention" tour are a case in point, he noted.

The music industry overall is having a tough time amid falling CD sales and rising iPod sales. The live concert business was the one form of music delivery that seemed immune to competition from the Internet and downloaders.

But now a stool and a microphone are no longer enough.

Ticket brokers and industry observers say two related forces are at play. One is the sheer number of crooners young and old now criss-crossing the country. Factor two: rising ticket prices.

Issac, the Las Vegas ticket broker, said he hasn't seen such a full concert lineup in about a decade, with the likes of David Bowie, Alicia Keys, Sting, Dave Matthews Band, Norah Jones and Prince still on deck for summer performances.

Solid ticket sales for Prince (top right) won't salvage a weak summer concert season.

Even with Aguilera and Spears out, hordes of young groupies aren't necessarily flocking to other artists. In fact, the only top-draw performer whose tickets are selling briskly is Prince, according to Issac.

Having so many acts is "too much of a good thing," said Issac. "People have only so much money. They can't do everything."

It doesn't help that ticket prices are soaring. Since 1996 the average cost of a concert ticket has nearly doubled, to $50.35 last year, according to Pollstar.com, the magazine's online edition.

Leading the way is the Material Girl herself. The average face value for a Madonna ticket is $175, according to Pollstar, and in Vegas, where concert ticket prices usually top out, promoters were asking $375 for the best seats.

"We keep pushing the envelope on ticket prices and I think it's finally gone beyond the pale a little bit," said Pollstar's Bongiovanni. "Consumers are voting with their wallets and they're keeping them in their pockets."

To be sure, the concert industry isn't on life support. Overall concert ticket sales reached $2.5 billion in the U.S. last year, up from $1.7 billion in 2000, according to Pollstar.

But those seemingly rosy figures mask who's making -- and who's losing -- money.

Unlike with album sales, the performers themselves pocket the bulk of the price of a ticket, portions of which pay for five-star hotels, bodyguards and so on. So when a rock star cancels her tour, her bank account feels the pain too.

For promoters, who get a much smaller slice of ticket sales, what matters most is concert attendance. That's because they make most of their money off T-shirt sales and other services like parking.

And fewer ticket sales can mean fewer sales of $85 Material Girl rhinestone camisoles.

In recent years growth in ticket sales has been fairly anemic.

After falling below 2000 levels for two years in a row, the total number of Top 100 concert ticket sales reached a record 38.7 million last year, according to Pollstar.com.

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Concert promoters say they've gotten the message -- hence, this month's "$10 ticket Tuesday" Clear Channel offer in San Antonio and a similar fire sale in San Francisco. More promotions are likely.

"I think we do need to move very swiftly in creating value for consumers and reacting to the economics," Brian Becker, CEO of Clear Channel Entertainment, told CNN/Money. "What consumers are saying is that they love the music and they will come out but they need a better value proposition." CCE is owned by San Antonio-based Clear Channel Communications. (CCU: Research, Estimates)

It's rare that concert promoters openly admit that a tour was aborted due to bleak sales.

But that's what the producers of "Lollapalooza," the two-day traveling alternative rockfest that got its start in 1991, said when they pulled the plug this week on a 16-day tour set to begin in July. In its heyday, the festival featured the Beastie Boys, the Red Hot Chili Peppers and other hot bands.

"Our plight," Lollapalooza co-founder Perry Farrell wrote on a tour Web site, "is a true indication of the general health of the touring industry and it is across musical genres."