Mobile CPMs are now averaging $15, compared to the $20- to-$ 25 average earlier this year, Advertising Age reports. "It seems to be part of a steady decline rather than a single event that sent CPMs tanking," said Eric Bader, managing partner of Brand in Hand, a buyer of mobile media for brands like Procter & Gamble and General Mills.

However, CPMs are not falling rapidly in every category. One area that still demands higher costs is targeted placement on a wireless provider's deck that accesses the mobile web.

Pricing for high-traffic sites and channels like weather, news and sports still demand premium CPM levels, but social media sites with a high level of inventory often price below $5; less-established sites will also have a lower asking price.

Despite the general trend of falling mobile advertising costs, mobile impressions remain a more expensive venture than online because they offers better returns; mobile boasts a clickthrough rate of about 1.5 percent, compared to 0.15 percent for online.