I recently used Xamarin to create and publish Android and iOS versions of a chess based game (ChessFlash KnightVision) and thought that I would share my metrics for code reuse across the platforms. This is based on the most current versions on my development box (newer than the released versions) and is based on lines of code in my *.cs files.
51% Common Code
20% Android Specific Code
29% iOS Specific Code
The game itself is described and shown in video at: http://chessflash.com/mobileapps.html
There is more iOS than Android code because the iOS version has separate code for some iPad vs iPhone aspects like split view controller and more orientation changes. As I add to this app and create other chess-related apps I expect that the percent of shared code will grow.