American Idol
American Idol became the most expensive series on broadcast networks for advertisers starting season four, and by the next season, it had broken the record in advertising rate for a regularly scheduled prime-time network series, selling over $700,000 for a 30-seconds slot, and reaching up to $1.3 million for the finale. Its ad prices reached a peak in season seven at $737,000. Estimated revenue more than doubled from $404 million in season three to $870 million in season six. While that declined from season eight onwards, it still earned significantly more than its nearest competitor, with advertising revenue topping $800 million annually the next few seasons. However, the sharp drop in ratings in season eleven also resulted in a sharp drop in advertising rate for season twelve, and the show lost its leading position as the costliest show for advertisers. By 2014, ad revenue from had fallen to $427 million where a 30-second spot went for less than $300,000.
In which season did American Idol become the costliest show for advertising on television? 
season four
American Idol became the most expensive series on broadcast networks for advertisers starting season four, and by the next season, it had broken the record in advertising rate for a regularly scheduled prime-time network series, selling over $700,000 for a 30-seconds slot, and reaching up to $1.3 million for the finale.
American Idol