Most readers of this blog are already convinced of the cost-effectiveness of search marketing. Just read an article from Reuters that talks about the relative value of search engine marketing compared to other forms of advertising. These numbers are presumably from paid search (ppc) -- organic (natural) search would likely be an even lower cost per new customer acquisition cost.
According to Piper Jaffray, the cost to acquire a customer is about $8.50 for search, $20 for Yellow Pages, $50 for online display ads, $60 for e-mail and $70 for direct mail. Data for television was not included.
Three cheers for search engine marketing!!!