The Super Bowl is the world championship of American football, but it's also US culture's premiere marketing opportunity. The 21 most-watched television programs in American history are all Super Bowls, and the 2015 Super Bowl is expected to make the list as well. This exposure comes at a price, $4.5 million for a 30-second spot, which naturally leads to the question: is it worth it?

The answer, this year, may be no.

Pay per viewer

The easiest way to look at this question is to start with industry-wide CPM statistics. That stands for cost per mille, the price an advertiser pays to reach a thousand viewers (advertisers love Latin, apparently). The average CPM for 2015 so far is $37.35 which makes a $4.5 million Super Bowl ad worth the cost if it gets 120.5 million viewers.

How plausible is that? Well, last year's Super Bowl secured 111.5 million US viewers which was up from 108.6 million the previous year.

An increase of 9 million isn't inconceivable. The population is growing, the matchup is arguably a bit more historic (Seattle is going for a rare repeat championship, New England is aiming to cement its legacy as the dominant franchise of our era) and the game is expected to be very close. But it is certainly an aggressive viewership target that would require a spectacular level of growth over and above the Super Bowl's already spectacular viewership level.

From bargain to premium

As this great chart published last year by Eric Chemi in Bloomberg shows, the price of Super Bowl ads has been growing much faster than Super Bowl viewership:

This means that the Super Bowls ads of yore, though extremely expensive in absolute terms, were probably bargains as means of reaching a large audience. Many companies, perhaps, simply weren't willing to pony up the enormous sums involved — creating an opportunity for a handful of mega-brands to buy viewers in bulk. But nothing lasts forever, and the continued pace of appreciation has now created a situation where Super Bowl advertisers are now paying a premium in CPM terms.

The Super Bowl of marketing opportunities

Of course this doesn't prove that a Super Bowl ad is a bad idea. Even if it's not the most cost-effective way to reach 100+ million Americans, it's the only way to reach a population of that size simultaneously.

It's that super-sized one-shot audience that's long made the Super Bowl a great platform for rolling out brand new products or marketing campaigns. There's something unique about seeing something for the first time, and the Super Bowl maximizes the number of people who'll do that all at once. Brands inclined to pay a premium for the Super Bowl opportunity can no doubt talk themselves into the view that the unique nature of the event makes it worthwhile.

Whether this is really true or not is hard to say. What is clear, however, is that advertising spending is pro-cyclical. In other words, when the economy grows ad spending grows even faster. During recessions when the economy shrinks, ad spending shrinks even faster. This winter, we're enjoying stronger economic growth than we've seen in years, so it's no surprise that companies are willing to get more aggressive with their ad dollars. If growth keeps up through 2015, we should expect ads at next year's Super Bowl ads to sell for an even more eye-popping figure than this year's $4.5 million.