From Pinch Media in November 2009:

• The average of paid Apple iPhone App Store applicaitions was downloaded 9,300 times.
• The most successful 10% of all paid apps, are downloaded over 75,000 times each.
• The second decile or 20% most successful percentile does 9,232 downloads each.
• The 30% most successful iPhone apps, they achieve only 3,856 downloads
• The median (the real half point), meaning that half of all paid iPhone apps achieve this level, and half do not achieve this level – is ‘under 1,000′ downloads.
• The figures given at prMac in November 2009 that the costs of an iPhone App ranged from 15,000 dollars to 50,000 dollars,
• According to Appsfire, 80% of all iPhone apps get “barely any active users” (this includes free and paid apps).
• The cost to buy the visibility or your App to try to get into the Top 25, would cost you 10,000 dollars in October 2008 soon after the App Store had launched. It shot up to 20,000 dollars just half a year later by March 2009 and today costs 250,000 dollars.

From: http://communities-dominate.blogs.com/brands/2010/03/paid-smartphone-apps-half-get-under-1000-downloads-what-does-this-mean.html