How efficient can Blog-Advertising be ?

Just finished listening to the latest Across the Sound podcast from Steve Rubel and Joseph Jaffe. Among other interesting things, they talked about an intriguing stat from a recent Audi campaign. Joseph blogged about it back in October, but I overlooked it some how then:
On Wednesday at OMMA East, GMD Studios CEO Brian Clark, whose agency does work for Audi, said, on a panel, that 29% of traffic to a site created as part of a recent Audi A3 campaign was generated by advertising on the BlogAds network. The kicker is that 29% was achieved with just one half of one percent of the overall media budget.

To drive the point home even further, McKinney + Silver, on its A3 timeline site states, "The media cost for the entire blog ad buy was less than the cost for one banner ad on a mainstream site such as Yahoo!"
Blogvertising got Audi 29% of traffic for this campaign with just one half of one percent of the overall media budget ! Any questions left ?