In campaigns that solicit work from the public, the model appears to be quite different — consumers, after all, create the ads. But, in reality, ad agencies and brand marketers are still doing much of the legwork. Heinz and Doritos spent months planning their user-generated contests, hiring lawyers to vet them and designing advertisements to promote them. Then they assigned employees to wade through entries.

“These contests have nothing to do with cost savings,” said Jared Dougherty, a spokesman for Frito-Lay, the division of PepsiCo that owns the Doritos brand.

While the winners of the Doritos contest may have spent only $12, Doritos spent about $1.3 million on advertising in October, according to estimates from Nielsen Monitor-Plus. And that was when it was promoting the contest, which invited people to create a 30-second commercial that would run during the Super Bowl. Doritos received 1,020 videos and awarded prizes of $10,000 to five finalists.

And then Doritos, a unit of the Frito-Lay division of PepsiCo, spent more than $8 million on advertising in February when it showed the top five commercials, more than any month in the last two years, according to Nielsen Monitor-Plus.

Other companies are also spending handsomely to present user-generated content to the public. Last Tuesday, KFC put on a commercial during “American Idol” that consisted entirely of clips about KFC that consumers had posted on the Internet — even without a contest. Heinz, too, says that customers have been making videos starring its bottle long before its contest and posting them on sites like YouTube.

Heinz has run ads for its contest during “American Idol” and other television shows (as well as in large newspapers like The New York Times), but it has gone a step further: it has converted all the labels on its bottles and ketchup packets into ads for the contest. This was a major initiative that involved everything from building new industrial printing plates to timing the shipment of bottles so they would appear on shelves at the beginning of May, said Mr. Ciesinski of Heinz.

And for all of Heinz’s effort, the interests of many of the contestants lie far outside its own. Steve Sass, 48, who taped two Heinz commercials, is running for president as a write-in candidate. Ed Barry, 34, writes sketches about a character named Vinny and is trying to get his work noticed. Some contestants say in interviews that they prefer mustard or mayonnaise.