App Store business model shifting to advertising 

Starting in 2012 advertising in my apps made up around 10% of sales whereas now it is nearly 80%. That increase has come almost entirely from a near collapse of my paid upfront sales (with my in-app purchase income largely unchanged).

David Smith, one of the most successful indie app developers has been extensively tracking his income sources over the past four years and has some great findings in his Evolving App Store Business Models article.