Whenever I give a classroom talk about the TV industry, I start by drawing a big dollar sign on the blackboard.

That's because most of the moves in TV, no matter howe boneheaded, are usually traced to money.

For example: Want to know why Two and a half Men star Charlie Sheen can command a reported salary of $800,000 per episode? Because he's the biggest star on the show with network TV's fourth-highest ad rate, commanding $226,635 per 30 seconds.

Why did NBC cancel the cop drama Southland before it even aired a single episode of its second season? because Dateline NBC's 9 p.m. Friday edition -- where Southland would have aired -- gets $55,270 per 30 seconds, just $4,000 behind 20-year cop series Law & Order at 8 p.m. same night.

Which means Southland wasn't going to earn much more in ad revenue, despite per episode costs which might be double Dateline's expenses.

In fact, one look at Friday's prime time ad prices and you can see why the networks care so little for shows airing that day. No show earns more than $100,000 per 30 seconds in revenue and only CBS' highly rated lineup cracks $70,000. And even though producer Barry Levinson can't understand why the networks don't put new shows on Saturday night, I  know; no advertising rate on Saturdays rises above $46,000, except for college football on ABC at $140,080.

Those are a few of the harsh lessons in the lineup of 30-second ad rates revealed by Ad Age magazine for the network's prime time shows. Among the lowest rates in the top four networks is Jay Leno's 10 p.m. show, which earns a high of $65,678 on Tuesdays and a low $48,803 on Fridays.

The big winners are NBC's Sunday Night Football, with the top fee at $339,700, followed by Grey's Anatomy at $240,462, Desperate Housewives at $228,851 and Sheen's sitcom. Sundays are big TV nights because they attract the most viewers, Thursdays are important nights because of advertising for weekend events such as movie openings.

Newcomer FlashForward earns early twice the rate of CBS' venerated 60 Minutes ($175,724 compared to $93,772. The Simpsons still earns $201,920 after 20 years. And, probably because it airs at 10 p.m.,  ABC's DOA drama Eastwick still earns more ($110,558) than better-regarded fare such as Bones ($107,942), NCIS: Los Angeles ($106,508) and Law & Order: SVU ($101,632).