A lot of marketers work using simple math. I spent this many dollars on an ad, it drove this many visitors to my site, and this many dollars in total sales. I can calculate a simple ROI and determine whether or not that ad or that campaign was successful and worth repeating or expanding.

And most times, that’s how it works.

But what about a marketing promotion that is designed to create buzz? What about the campaign that does something immeasurable?

On Saturday, I posted this comment on using commercials to attract attention, using JCPenney’s recent ad to highlight the notion of attracting attention over and above driving sales. It’s a great example of money being spent on advertising that aims to do more than just get men to buy JCPenney clothing.

HBO just upped the bar. Last week, HBO reportedly paid the MTA $150,000 to run a 1920s-era vintage subway train to promote the upcoming 2nd season of Boardwalk Empire. Nothing was done in advance to promote that this was happening, it just did. Subway passengers lined stations as they normally do, and were greeted by a classic subway look, decked out in promotional materials for the show and the characters in it.

So how do you measure performance on a promotion like this? Do you look at the number of new HBO subscribers in the weeks after it runs? Do you look at DVD sales and rentals for the first season?

In cases such as this, the $150,000 promotion is aimed at accomplishing something much larger than just sales. It creates a media buzz around the show. It’s new and different, worthy of press, and in need of spreading. People take photos and share them. Bloggers like me get a hold of them and share the story.

And suddenly, everyone is talking about you.