This is from a recent report in Advertising Age:
Marketers will pony up as much as $700,000 for a 30-second spot in Fox's midseason hit, "American Idol," according to an initial survey of media agencies, conducted annually by Advertising Age. The Tuesday-night singing competition, returning in January, is the most expensive show for the third year in a row. Back in 2003, when NBC's "Friends" topped the list, "American Idol," didn't even make the top ten. The going price for 30-second spot on Fox's wildly popular 'American Idol' ranges from $500,000 to $700,000. ... Others at the top of the heap include Fox's "House," ABC's "Desperate Housewives," and ABC newcomer "Brothers and Sisters.''
You can find the full report here.