Ok, so this isn't really a new application - I simply re-wrote the previous application over as a WPF Application. Same functionality (although this obviously breaks Mono compatibility).
I will say, I do like working w/ WFP better than Winforms from the GUI design aspect - XAML seems pretty straight-forward.
Like BillCalc, this is hosted on Github as well under the name of BillCalc2.
Here are a couple of Gists for you.
First, the MainWindow.xaml.cs file doing all of the work:
And of the XAML for the Main Window: