Let’s face it. Most rate-card prices for renting mailing lists, print ad space, interactive, and radio and TV air time have some built-in wiggle room, but what you’re likely to get charged for broadcast and interactive media is subject to the most fluctuation.

To understand why, it’s easiest to compare buying broadcast and interactive time with buying an airline ticket–though we believe there’s far more logic in the way most commercials are priced.

Think of these media outlets as an airline. Now, think of each program or site as a specific plane flight. There are so many allocated commercials per day on these media, just as an airline will have a certain number of flights each day. And, there’s a specific amount of commercial time and space allocated, just as a particular plane flight has a particular number of seats.

Commercials on the most popular media outlets command the highest prices, are scooped up more quickly, and are subject to little negotiation. But, if you commit for them as early as possible–often based on faith with what could be a hot new show or site, you have a better chance of getting a spot–and perhaps at a discount. Of course, as it gets close to air time and there are unsold spots, you may end up with a last-minute bargain. (Think about standing by to try to get on a certain flight.) But, if it’s vital your spot is on a particular program or site, you’re advised to buy as early as possible.

However, you may not even be able to cancel if you change your mind, or you might be subject to stiff penalties–even if your circumstances are altered.

Quantity also plays a big part. (Imagine booking 50 people versus one on an airplane.) If you’re willing to buy a season’s worth of commercials or the entire ad space on a home page, you’ll end up way ahead of an advertiser who wants only a few spots. In fact, media outlets prefer to wait for the big commitments before they sell the small ones, or they could lose a big sale because they weren’t able to deliver what the deep-pocketed advertiser desired.

As you get closer to flight time, there are less and less seats left, and the prices for each remaining seat increases. Early shoppers are rewarded, and last-minute buyers are penalized. That’s why it pays to schedule as far in advance as possible, hopefully before others express their desires.

If it doesn’t matter which show or site your commercial airs on (a questionable decision in our opinion), then you can get great buys on “remnant” time. Again, it’s like a person who just wants to fly from here to there, but is willing to fly on any day at any time–and not necessarily from the most convenient airport. Or if you’re willing to buy a package which will contain some prime spots plus some others you could care less about, you’ll get a lower cost-per-spot. These are often great buys, but you shouldn’t be deluded by artificially low costs because part of what you’re buying is likely be something that could have been sold at only rock-bottom costs, if at all.

Media planning and buying for broadcast and interactive is not for the amateur. Agency media people do this day after day, year after year. They can run amazingly sophisticated reports, matching the demographics of your desired target audience to the best spots available. (Do you, for instance, want young mothers with a family income of $50,000+, who have a college degree, are working full-time outside the home, and belong to an HMO?)

They understand all sorts of statistics necessary to plan a successful campaign such as reach versus frequency. Plus, the agency media pro knows who’s cutting what sort of deals–and how best to compete for them. And, even if they can’t negotiate a lower price for you, they may be able to add some great merchandising extras to your buy.

So, when it comes to buying air time or interactive, don’t err. You may think you’ve pulled a fast one by yourself, but you may realize you got your great deal because you’re flying in the cargo hull.