Why is StringFormat in XAML giving me odd results?

I have n Entry bound to a property in a viewmodel that is a double. I use something like: Text = "Binding TBValue, Mode=TwoWay,
StringFormat = '{0:C0}'}" but if I type 1200 in the entry only the number 1 is saved in the viewmodel rather than 1200. Any idea as to why this is?

Thanks