all, I'm thinking of porting my application from VB.net to the C# based MONO project, so it can run on both Windows and Mac. However, I am in need of a Mac-friendly alternative to WPF. It has to have very similar functionality. QML (by QT) is not a viable option, as it costs far too much money for us.
Miguel de Icaza recommends MonoMac as a substitute to WPF for Mac as stated here. As he explains there is no plan to implement WPF in mono.
MonoMac is a work-in-progress. Some parts of the Cocoa APIs are not wrapped yet. You should start with the samples to get an idea of what is possible.
You will have to build two different UI on top of a common engine, but that's generally a good separation of concerns.
There is also a possible common code base for UI with GTK#.