The combined value of paid apps, app-enabled purchases of goods and services and in-app advertising is expected to double to $151 billion in the U.S. by 2017, AppNation said today. The research shows that the largest segment of the so-called app economy is sales of app-enabled physical goods and services, which made up about $45 billion of the nearly $60 billion total at the start of 2013. By the middle of this year, the $60 billion total reached about $72 billion, which is expected to more than double to $151 billion in 2017.