Tablet ad effectiveness rates climbed 31% in 2012, and by the end of the year, ad conversion rates on tablets was 3.3% compared to 3.9% on desktops. The idea is that by the end of 2013, search-ad prices for tablets are expected to pull even with desktop ad prices. Right now, tablet search-ad prices are 17% lower and climbing steadily.
Couple rising ad prices with the record sales of tablets, and it comes out to be a pretty big number. In 2012, mobile devices generated almost $8 billion in ad revenue for Google, and Marin projects tablets alone to generate $5 billion in 2013. That makes it seem like a pretty solid investment for Google to pay whatever it does to be the default search engine in iOS, since the iPad still holds close to half of tablet sales.