Android apps cost more than iOS apps
canalys.com
canalys.com
Also, there's a higher percentage of free apps on Android than on iOS. I very much doubt that the average app on Android is more expensive than on iOS, especially if you count iPad apps, which are usually twice as expensive as normal iPhone apps.
I wonder if perhaps it might be partly due to some nicely priced niche apps being better represented in (by definition more open) android market than in the iOS one. Not that I know of any or actually looked into it; plus, I doubt many such apps would get into the top 100.