It is TV ads which keep a show afloat and these shows manage the most money for ads in a half hour of air time.  It is also this reason that these shows continue to stay on the air while other shows whither and burn, getting canceled left and right.  In fact, those shows which are popular online, but fail to make money on air, usually get canceled, due to the lack of funding to keep the shows going.

A tv.com estimate of how much money the most popular tv shows make in a half hour:

1. American Idol–$7 million
2. 24–$3.7 million
3. Desperate Housewives–$2.9 million
4. Grey’s Anatomy–$2.7 million
5. Two and a Half Men–$2.6 million
6. Dancing with the Stars results show–$2.5 million
7. Dancing with the Stars performance show–$2.4 million
8. Heroes–$2.1 million
9. Brothers & Sisters–$2.1 million
10. Survivor–$2.1 million
11. CSI–$2.1 million
12. Extreme Makeover: Home Edition–$2 million
13. Worst Week–$2 million
14. House–$1.9 million
15. Private Practice–$1.8 million