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?