WPF Multibinding到另一条路径？

Question

这是我在ViewSource上绑定的DataGrid：

   <DataGridTemplateColumn x:Name="sumColumn" Header="Summary" Width="SizeToHeader">
    <DataGridTemplateColumn.CellTemplate>
        <DataTemplate>
            <TextBox>
                <TextBox.Text>
                    <MultiBinding Converter="{StaticResource SumConverter}" UpdateSourceTrigger="PropertyChanged" Mode="TwoWay">
                        <Binding Path="quantity"/>
                        <Binding Path="price"/>
                    </MultiBinding>
                </TextBox.Text>
            </TextBox>
        </DataTemplate>
    </DataGridTemplateColumn.CellTemplate>
   </DataGridTemplateColumn>

如何将CollectionSource中sumConverter的结果绑定到Path="sum"？