ollow the money is as good a rule in search as it is in politics. That's why thinks it's a smart move to focus on shopping.

Three-year-old began as Coupon Mountain, letting consumers search for discount coupons they could use at e-commerce sites. It's grown to become a comparison shopping site that helps users through the decision process then sends them off to e-tailers when they're ready to buy -- with a coupon, if one is available.

It turns out that general Web search tends to follow the 80/20 rule: A small proportion of all Web searches generate most of the ad revenue, according to co-founder Harry Tsao, who helped create Overture's pay-per-click marketing service.

"The search marketing industry today is $6 billion, but the ad revenue for [comparison shopping sites] probably doesn't even equate to $400 million," he said. "Yet we all play in an area that represents [about 80 percent] of the monetization."

Because of that, Tsau said, "Vertical search is set to explode."

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