The highest-priced keyword in the United States last month sold on Google for $99.44 per click, according to the AdGooroo Search Engine Advertising Update: Q309.

The report released Wednesday pegs Mesothelioma as the highest-selling keyword in September. The same word sold on Yahoo in the No. 1 spot for $60.68 per click. The phrase "auto insurance comparison" took top honors on Bing, bringing in $55.20 per click.

It's the first time that AdGooroo's quarterly report has analyzed the price for keywords, according to Rich Stokes, founder and CEO of AdGooroo. He couldn't tell Online Media Daily how many times the top keywords were clicked on during the month.

As for the word "mesothelioma," it seems lawyers have ramped up paid-search ads based on lawsuits related to the asbestos-causing lung cancer. The paid search ads direct people to lawyers affected by the death sentence. "If I was a law firm specializing in mesothelioma lawsuits I would put the paid-search ads on Bing," he says. "The word hasn't caught up demand on Bing as it has on the other two search engines."

Stokes believes average cost per click (CPCs) will remain flat through 2010. Advertisers can expect to see keywords shift in and out of the top-priced list. It suggests the industry has begun to mature.

The report also lists the top 25 U.S. advertisers by search engine. Among the top in alphabetical order are Amazon.com, Ask.com, Att.com, eBay.com, Google.com, Priceline.com and more. Companies come and go from this list each quarter, Stokes says. Travel companies seem to dominate in the first quarter, replaced in the fourth quarter by retail.

The study suggests that search engines have differentiated themselves in advertisers' minds. On Google, AdGooroo saw new entrants among branded retailers, such as Wal-mart Stores, eBay and Nordstrom. Yahoo's less expensive CPC prices lured several branded comparison shopping engines, including Expedia, LowFares, BookingBuddy, and Calibex.

Bing primarily picked up unbranded comparison engines, such as Nextag, k8yel.com, bestcompare.net, shopwhiz.net, and underpay.biz. The report states: "many of the top advertisers appear to be rebranded variations of the same site, suggesting that Bing is being used as an inexpensive marketing sandbox."

In September, the search engines served up on average between five and six ads per keyword. In the United States, Google moved to 5.45 in September 2009 -- up from 3.06 in the year-ago month. Yahoo dropped to 5.35 from 7.53; and Bing, 3.10, from 5.12, respectively.

It appears that the search engine relaxed some of the quality controls to let more advertisers back on the page, Stokes explains. "During the last year the search engines tried to figure out the optimum tradeoff between ads shown and revenue gained," he says. "In 2007, they thought dialing back the number of ads shown would produce higher click-through rates."

Stokes also points to consolidation in the paid search industry in the coming quarter as the industry continues to mature. Paid search has evolved less rapidly in the past two years, which has made it easier for large advertisers to adjust and adapt to the technology. Companies are throwing in more ad dollars to fewer companies. Budgets could move from the agencies back to the advertisers who have mastered paid search strategies.

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