Buying a 30-second spot on Fox's American Idol will set advertisers back as much as $700,000 when the show debuts at midseason, Advertising Age reported today (Friday), citing a survey of media agencies. It marks the third consecutive year that Idol has held the top spot as the most expensive regular series for advertisers. The trade publication observed that some marketers will pay significantly less for a spot, when the show returns in January -- depending on the number of spots they buy on it throughout the season and their overall ad commitment to the Fox network. Also, commercials running on the final nights of the talent contest will cost considerably more than those running at the beginning. Fox's House ranks as the second most-expensive regular show, followed by ABC's Desperate Housewives and NBC's Sunday Night Football.