<Grid>
    <Grid.Resources>
        <custom:ValueConverter x:Key="valueConverter"/>
    </Grid.Resources>
    <DataGrid>
        <DataGrid.Columns>
            <DataGridTemplateColumn Header="Summe">
                <DataGridTemplateColumn.CellTemplate>
                    <DataTemplate>
                        <TextBlock Text="{Binding Summe,Converter={StaticResource valueConverter}}"/>
                    </DataTemplate>
                </DataGridTemplateColumn.CellTemplate>
            </DataGridTemplateColumn>
        </DataGrid.Columns>
    </DataGrid>
</Grid>

public class ValueConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        int myValue = System.Convert.ToInt32(value); //or decimal
        //errechnen des neues Wertes
        return myValue ;
    }
    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}