Google’s earnings release Thursday for the second quarter, which ended in June, provided more evidence of the problem that the price advertisers pay each time someone clicks on an ad — or “cost per click,” in Google talk — dropped 6 percent from the year-ago quarter, largely because of the shift to increased mobile advertising. Mike Isaac reports that is in line with Google’s two-year trend of declining ad prices, while noting that Google’s “advertising competitors are still small in its rearview mirror.”