Consumers will spend more than $4.2 billion on apps in 2013, up from $343 million this year, the research firm says, as smartphones become ubiquitous and app prices rise.

The average smartphone owner "downloads about 20 apps per year," says Carl Howe, director of consumer research. "It's a bigger market than a lot of people have been thinking."

$4.2 billion by 2013 is a very impressive number.