Consumers were exposed to paid search ads on Google billions of times in 2010, and 8.4% of those impressions were on a mobile device, a 238% increase over 2009, according to a study by Performics of more than 175 of its retailer and consumer brand manufacturer clients. Performics is the performance marketing agency of Publicis Groupe’s ZenithOptimedia.

Mobile paid search impressions continued to increase for Performics’ clients in January, when 9.4% of total Google paid search impressions were in the mobile channel. Trends indicate that March will be the first month when more than 10% of all impressions come from mobile devices, the company says.

Mobile paid search clicks jumped from 7.6% of all clicks in November 2010 to 9.2% in December. Click share decreased slightly in January to 8.9%, driven primarily by a small group of clients that had significant December peaks in mobile clicks, Performics says. At this rate, the mobile share of total Google paid search clicks, like impressions, is likely to surpass 10% in March, the company says.

In December the cost per click for a mobile paid search ad was only 43.2% of that for a desktop PC paid search ad. December was the seventh consecutive month that mobile cost per click decreased for Performics clients. Performics declines to give the average cost per click for its clients, but says costs vary widely: from 5 cents to $10 for desktop PC search ads and from 3 cents to $5 for mobile search ads. Rimm-Kaufman Group LLC, an e-commerce consulting firm that specializes in paid search, says the average cost per click on Google currently is around 45 cents, noting that it had jumped to 62 cents during the holiday season. Combining Performics’ assertion that mobile paid clicks cost 43.2% of PC web clicks and the Rimm-Kaufman current desktop PC average of 62 cents per click, that would make the current average mobile cost per click about 19 cents.

“Going into 2011, mobile search is a must for retailers. If a retailer continues to avoid mobile paid search, it better be comfortable ignoring potentially 10 percent of its paid search market,” says James Beveridge, senior analyst, planning and analytics, at Performics. “Retailers that have been managing mobile campaigns independently for awhile have an advantage—the longer a retailer has been testing and optimizing mobile search campaigns, the higher click-through rates and lower costs per click they’ll realize. However, even new participants can benefit from the rapidly expanding and, for the time being, undervalued channel.”