That's actually not what we see with our customers and their apps.

Here is a real-world example of code sharing percentages for an iOS, Android, and Mac app written in C#: http://lipsky.me/blog/2012/9/11/touchdraw-code-reuse-updated

And here is another one across iOS, Android, Mac, and Windows, also in C#: http://praeclarum.org/post/31799384896/icircuit-code-reuse-t...

Both of these apps are using 100% _native widgets_ for their user interface, and I think it's fair to say that both of them are fairly UI-heavy. And yet they average >70% code sharing.

It should also be pointed out that these apps were written from scratch. In fact, Jon Lipsky didn't even know C# before he started writing TouchDraw (the first app I linked).