Bill O'Reilly nailed it a few weeks ago.

Ranting about a new teen drama on MTV, O'Reilly pronounced the show to be doomed. How'd he know? Simple, it had no sponsors. Explaining TV Economics 101, O'Reilly noted a television show “won't stay on the air if nobody would sponsor it.”

Turns out O'Reilly could have been talking about his soon-to-be former fellow Fox News host, Glenn Beck, whose program has been bedeviled by a laundry list of advertisers that won't touch his show.

Neither Fox News nor Beck are saying exactly why the two decided to break up this week. But if Glenn Beck enjoyed the ratings that it currently posts and was able to secure big-time advertisers, I suspect the messy breakup would have been averted. (Fat profits tend to help paper over differences.)

Instead, Beck may have learned a painful, supply-and-demand lesson: Revenues rule television, not just ratings. And as Fox News likely discovered, having a show like Glenn Beck that attracts two million viewers is great. Having a show with two million viewers that advertisers actually want to be on is even better.

Remember, Beck came up on the radio side where ratings truly are king and the quarterly Arbitron books are the definitive markers of success (professionally and often personally; radio guys are obsessed). In the world of AM talk, strong numbers can pretty much always be turned into solid station revenues, both locally and nationally for syndicated programs. In the world of AM talk, the host's only job is to grab attention and keep the listeners. Monetizing that big rating, and turning the Arbitron numbers into profits, is more or less a sure thing.

And for his Fox show, Beck did what any successful AM talker would do, create controversy and build your base. Except he did both on steroids. He went from zero to a 100 on the crazy scale in a matter of days. Soon after debuting on Fox, Beck was spouting insurrectionist rhetoric and gaming out what the looming American civil war would look like under Obama.

And from there things just went down hill, content-wise.

But it worked, at first. Glenn Beck became a cable news meteor and reached the three million-viewer plateau in early 2010. But by this January, that tally had tumbled to 1.8 million; still a success, but a somewhat dimmed one. During the ratings tumble, advertisers abandoned the show, put off by the incendiary content of Beck's relentless name-calling and unhinged conspiracy theories.

Here's a taste of what Fox News' sales team was missing out on during the 5 p.m. time slot: Applebee's, AT&T, Bank of America, Best Buy, Campbell Soup, Clorox, ConAgra, CVS, Farmers Insurance Group, GEICO, General Mills, Johnson & Johnson, Lowe's, Procter & Gamble, Progressive Insurance, RadioShack, Sprint, State Farm Insurance, The UPS Store, Travelers Insurance, Verizon Wireless, Vonage, and Wal-Mart, among others.

All of which meant Fox News was left with a ratings success that it couldn't turn into big profits, which in turn meant hosting Glenn Beck represented a symbolic victory of sorts for Fox News. (We'll show those liberals!) But it was a hollow victory that came without blue chip invoices attached. Traditionally, Rupert Murdoch is not in the business of celebrating hollow victories.

Last month, AOL's Daily Finance published an eye-opening analysis of Beck's advertising base, as compared to his cable news competitors in the same time slot. Daily Finance found that Beck's show, during a one-week period, attracted 39 different advertisers, while MSNBC's Hardball boasted 59, and CNN's The Situation Room drew from more than 100 advertisers. CNN's 5 p.m. sponsors included Madison Ave. studs like Procter & Gamble and Microsoft. Beck's? SeniorPeopleMeet.com, a dating service for retired folk.

Yes, Glenn Beck has higher ratings than The Situation Room. But raise your hand if you think SeniorPeopleMeet.com paid more to advertise on Glenn Beck than Microsoft did to be on The Situation Room.

Yeah, me neither.

How much has the advertising exodus cost Fox News? In September 2009, ColorOfChange, which was instrumental in launching the Beck ad boycott, published its analysis. Based on advertising rates it concluded that Glenn Beck was bringing in approximately $600,000 less per-week (or approximately $2.4 million per-month), than it was before the boycott began. Keep in mind, that's when 50 or 60 advertisers had jumped ship. Today, that number hovers between 300-400.

Using that $2.4 million per month estimate, since the fall of 2009, it's possible the ad-starved Beck show booked nearly $43 million less than it would have if it weren't facing a boycott. $43 million.

It's true that throughout the boycott, the official word from Fox News has been that the advertising exodus wasn't a big deal and that they were just moving advertisers around to other programs and that everything was just fine on the sales side at Glenn Beck. All of which made no sense.

The television industry is built around supply and demand. Glenn Beck has supply in the form of roughly 20 minutes of advertising time sold each episode. And it wants to build demand. Usually, healthy ratings drive that demand since advertisers want to reach the masses. But if suddenly hundreds of advertisers raise their hand and announce they'd be happy to spend money with Fox News, but not on Glenn Beck, then the show's demand plummets, but the supply -- the 20 minutes of advertising inventory -- remains the same.

Bottom line? The ad rates go down, even if Glenn Beck remains the third highest rated show in cable news.

As Brad Adgate, director of research for the ad-buying giant Horizon Media, noted in an email to me yesterday [emphasis added]:

Ad rates are predicated on supply and demand, not ratings. If the show has low demand and an oversupply of advertiser inventory, the show will not get premium ad pricing no matter how many viewers. >

Note that ratings are of secondary importance to revenues. That's a lesson Beck may have just learned the hard way.