Установленное значение к пустому указателю в привязке WPF

This value converter should do the trick :

public class StringToNullableDecimalConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, 
        CultureInfo culture)
    {
        decimal? d = (decimal?)value;
        if (d.HasValue)
            return d.Value.ToString(culture);
        else
            return String.Empty;
    }

    public object ConvertBack(object value, Type targetType, 
        object parameter, CultureInfo culture)
    {
        string s = (string)value;
        if (String.IsNullOrEmpty(s))
            return null;
        else
            return (decimal?)decimal.Parse(s, culture);
    }
}

Declare an instance of this converter in the ressources :

<Window.Resources>
    <local:StringToNullableDecimalConverter x:Key="nullDecimalConv"/>
</Window.Resources>

And use it in your binding :

<TextBox Text="{Binding Price, Converter={StaticResource nullDecimalConv}}"/>

Note that TargetNullValue is not appropriate here : it is used to define which value should be used when the source of the binding is null. Here Price is not the source, it's a property of the source...

Вы можете попробовать использовать ValueConverter (IValueConverter) http://msdn.microsoft.com/en-us/library/system.windows.data.ivalueconverter.aspx

Здесь, на затылке, что-то вроде:

public class DoubleConverter : IValueConverter
{
    public object Convert(object value, Type targetType, 
        object parameter, CultureInfo culture)
    {
        return (double)value;
    }

    public object ConvertBack(object value, Type targetType, 
        object parameter, CultureInfo culture)
    {
    var doubleValue = Convert.ToDouble(value);

    return (doubleValue == 0 ? null : doubleValue);
    }
}

(Может потребоваться небольшая настройка хотя)