Earlier this week, I attended an IAB debate about the value of mobile advertising, revolving around the question of whether it was too expensive.

My answer was that yes, it is. But my reasons are quite different to those that were debated on the evening.

"Mobile" covers a range of different channels, from SMS and MMS to email, to Bluetooth messages, to display advertising on websites (or WAP)- not to mention the possibility of using services like Facebook or Twitter. All of these can be used by businesses to deliver commercial messages to their audience- and each one is suited to a different kind of message with different goals, and at vastly different costs.

The big catch is that, as the "digital" world is discovering, the more personal a communications channel is, the more difficult it is to distract people from what they are doing with advertising. And mobile is about as personal a communications channel as you can get. The fact that people are so connected to their phones might sound like a positive factor, but when an advertising message is considered to be annoying or distracting, it becomes a massively negative factor. This is especially true in the case of brand advertising (where the whole point is to make someone like a brand), and means that if you want to compare the cost of reaching 1,000 people who fall within your target audience, there are factors beyond CPMs that have to be considered.

Yet there are plenty of other ways to reach an audience via mobile; those who opt-in to be a fan on Facebook, to follow a brand on Twitter, or to receive email newsletters to name a few. These are all incredibly targeted and effective, and very cheap in terms of the money that has to be spent. Most importantly for the networks, media owners and agencies that want to reach an audience through their mobile, they don't drive revenues.

What they can do though is add value; by using traditional advertising (and I'm including online advertising in this broad definition) to drive people to opt-in to these messages, the value of other media is increased. An outdoor poster that invites people to send an SMS message to ask for information, or an MPU that tells people about the news and information on a brands Facebook page or Twitter stream extends the message beyond the bought space.

So before debating whether "mobile advertising" is too expensive, I think there is a need to debate what mobile advertising actually is. Where does it live? What does it do? What can it do? What can it do well?

Then we can worry about how the mobile space compares to bought spaces on TV, poster sites and cinema. But by then, I suspect the idea of "expensive" will have moved past the money it costs and on to the work that needs to be done elsewhere.