Apple's iTunes app store generated approx $15bn in revenue in 2014

"Apple said total billings from paid apps and purchases made within apps for its mobile devices rose 50% last year, from more than $10 billion in 2013. That implies App Store sales of roughly $15 billion, making Apple’s take around $4.5 billion. Apple keeps 30 cents of every dollar that flows through the App Store.
That is still a relatively small slice of Apple’s $183 billion in annual sales. But it is growing much faster: Analysts estimate that Apple’s revenue grew 10% in 2014, according to CapitalIQ.
Moreover, the vast majority of App Store revenue flows directly to the bottom line—making it an increasingly meaningful contributor to Apple’s profit. Apple’s net income for the year ended in September 2014 totaled $39.5 billion."

Source:  WSJ, 8th January 2015