The cost to acquire a loyal user, or one that opens an app three times, grew from $1.30 in 2012 to $1.62 in 2013. Meanwhile, app downloads over 2013 broke records, though the download growth has been slowing, as previous reports have pointed out, indicating that some maturer markets are nearing their saturation point

http://techcrunch.com/2014/01/03/mobile-app-marketing-costs-hit-record-highs-in-2013/