Cheap mobile search ads likely continued to drag down the overall per-click rate advertisers paid Google in Q3, yet the trends look good for the search leader, analysts say. The price advertisers were willing to pay Google (GOOG) last quarter for mobile search ads was 40% to 50% below the same ads on desktop computers, observers say. But in prior quarters, the mobile ad rates -- called cost per click, or CPC -- were as much as 70% below PC rates,