Internet search is the most efficient marketing channel for customer acquisition, more than twice as efficient as yellow pages, which is next best, according to a Piper Jaffray study of five channels.

The cost per acquisition for search is on average $8.50, according to the report, "The New eCommerce Decade: The Age of Micro Targeting," released earlier this month, reports Internet Retailer. Yellow Pages was the second-most efficient channel at $20 per customer acquisition, followed by online display ads at $50, email at $60, and direct mail at $70.

The study also found that because of the growth of local search, the number of online advertisers could reach 2-4 million over the next five years from the current 700,000.