Query: In which season did American Idol become the costliest show for advertising on television? 
Context: 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.