Network TV: $20-40cpm per ad - 15-19 ads shown in 30 minutes
Cable: $1-15 cpm per ad - 10-22 ads shown (plus subscription revenue)
iTunes: $0.80 per viewer, so CPM of $80 total (est network
Hulu / Online: CPM $10-40. 1-4 ads per show.
If you do the math, its obvious that network broadcasts really rake it in. A 30 second spot on a prime time network show sells for anywhere from $400,000 to $800,000 per spot (for 15-30M viewers).
If everyone started watching Hulu instead, they would lose 80-90% of revenue. If everybody downloaded on iTunes (assuming 3 viewers per download, or more is likely) they would still lose out but not as much.
The problem is if the networks start offering a la carte shows for $2 a pop, they lose the bundling revenue (ie. channels you receive in a bundle but would never pay for on their own).
Supply and demand. If everyone started watching on Hulu they would be able to charge more per advertisement. These are rates given the current situation, not the future. Why do I say this? I watch Hulu. I do not watch TV otherwise. If the advertisers (and networks) want my eyeballs they need to provide content via Hulu.