Clicks on paid search ads cost 6 percent less for retailers than they did in May 2011, but advertisers actually managed to spend more (17 percent in fact) as consumers clicked on more sponsored listings.

SEM provider PM Digital’s May 2012 Rewind Index, which measures U.S. paid search performance for online retailers, suggests that while average CPC’s dipped in May (following 2-3 percent dips in immediately prior months), much of the declining activity can be attributed to mobile search campaigns, which according to PM Digital “represent a growing slice of the total paid search pie but which cost a fraction of regular search ads.”

And there’s more good news: revenue (at least for those merchants surveyed) was up a whopping 34 percent – although much of that increase can be attributed to some important holidays including Memorial Day and Mother’s Day.