When Republican presidential candidate Rick Santorum suspended his presidential campaign last month, the former Pennsylvania senator all but sealed Mitt Romney’s easy victory in the state’s April 24 primary.

Santorum also dashed the expectations of his home state’s broadcasters, who were counting on the candidate to keep the race competitive and their ad inventory—much of which had already been reserved by Romney’s campaign—in high demand.

The day after Santorum dropped out, The New York Times reported that “Romney aides immediately went to work canceling what they expected would be a $2.9 million advertising campaign in Pennsylvania, a huge savings equivalent to roughly 40 percent of the cash Mr. Romney had on hand at the end of February.”

The development led Kathy Kiely of the Sunlight Foundation to claim on the organization’s blog that the “biggest loser” in Pennsylvania primary was not Santorum, but the state’s 46 local broadcasters.

That got us thinking: we’re sympathetic to the argument that the windfall that local TV outlets gather during the political season ought to be returned, to some degree, to the public in the form of stronger political reporting.

But, given that there are only so many hours in which to sell ads—and other potential advertisers ready to buy that inventory—just how big is that windfall? And just who are its beneficiaries?

The answer, as with many things: it depends.

An ‘unprecedented frenzy,’ unevenly spread

Projections vary, but all observers agree that the 2012 campaign is likely to set records for TV ad spending. Citing a “near-perfect political storm,” a Moody’s Investors Service report issued last June predicted an “unprecedented frenzy of political advertising” and 9-18% revenue growth above 2010’s $2.3 billion, the current record. During an event for media buyers in New York last month, Ken Goldstein, president of Kantar Media’s Campaign Media Analysis Group, offered a higher ceiling, estimating $2.5-$3.3 billion will be directed to local spot advertising.

Those projections are based on the assumption that local television will attract a little less than half of all campaign dollars, as it has in the past. That assumption is likely to hold up, observers say. Even as campaigns diversify their messaging strategies and develop new approaches to the ground game, television remains the dominant way to reach potential customers—and swing voters.

But the distribution of those dollars is far from even. The bulk of the cash, of course, flows to stations in politically significant markets—presidential battleground states, or those with early primaries, or especially competitive Senate races.

Even within those markets, though, ad buys are heavily skewed toward top-ranked stations. Moody’s analyst Carl Salas says a market’s top broadcaster will typically draw 40-65% of the political spending, the second-rated draws about 30-40%, and the bottom two stations will split the balance, perhaps 5%. And while strong network programming boosts ratings and attracts ads, the greatest value lies in having the top local newscast, as the station owns that ad inventory and gets to keep 100% of the revenue it generates.

Just as the ads are concentrated by geography and station, they are also compressed by time. An analysis of 2008 and 2010 by a TV industry trade association found that political ad share builds from roughly 5% of all inventory around Labor Day to 20-25% in the days before the election. The shares are larger in battleground states. In the Harrisburg market, political spending accounted for 32% of spots and 48% of advertising revenue in the week before Election Day; in the Columbus market, political buys made up 50% of spots and 61% of revenue for that week.

All that concentrated demand puts pressure on prices to rise, but that doesn’t mean campaigns are paying top dollar. By law, candidates for federal office are entitled to rates that are the “lowest unit charge per class of time” within 45 days before a primary, or 60 days before a general election. That means they receive the discounted rates given to loyal, favored advertisers. (While the campaigns get the lowest rates in a given ad class, they do tend not to buy the cheapest class, which is subject to “immediate pre-emption.”) In fact, Goldstein and his Kantar colleague Elizabeth Wilner argue that the primary motivation behind broadcasters’ opposition to a new FCC rule mandating that political ad buys be posted online is that the broadcasters don’t want ordinary non-political advertisers to know just how low those rates are.

Erika Fry is a former assistant editor at CJR.