How to calculate totals of wpf datagrid columns which is filled by user data entry?

DataTemplate:

<!-- ProductName column -->

    <DataTemplate x:Key="NotEditableStateColumnTemplate">

        <Image Height="20" Width="20" Source="{Binding Path=NotEditable, Converter={StaticResource NotEditableStateConverter}, Mode=Default}" HorizontalAlignment="Stretch"/>

    </DataTemplate>

    <!-- Barcode column -->

    <DataTemplate x:Key="ProductBarcodeColumnTemplate">

        <TextBlock  FontFamily="Tahoma" FontSize="14" Text="{Binding Path=ProductBarcode}" HorizontalAlignment="Stretch" Foreground="#FF44544A" />

    </DataTemplate>

    <!-- ProductName column -->

    <DataTemplate x:Key="ProdNameColumnTemplate">

        <TextBlock  FontFamily="Tahoma" FontSize="14" Text="{Binding Path=ProductName}" HorizontalAlignment="Stretch" Foreground="#FF44544A"/>

    </DataTemplate>

    <!-- Price column -->

    <DataTemplate x:Key="PriceNameColumnTemplate">

        <TextBlock x:Name="SalePrice" FontFamily="Tahoma" FontSize="14" HorizontalAlignment="Stretch" Text="{Binding Path=SalePrice, StringFormat={}{0:#.##}}" Foreground="#FF44544A"/>

    </DataTemplate>

    <!-- Quntity column -->

    <DataTemplate x:Key="QuntityColumnTemplate">

        <TextBox x:Name="txtQuntity" Tag="{Binding}" PreviewTextInput="txtQuntity_PreviewTextInput" Style="{StaticResource TextBoxStyle}" Height="20"  Foreground="#FF0A421F" FontFamily="Tahoma" FontSize="14"  Text="{Binding Path=SoldQuantity}" HorizontalAlignment="Stretch"/>

    </DataTemplate>

MainWindow:

<myClasses:SortableListView Margin="9,6,6,0" Height="auto" BorderBrush="{x:Null}" Background="{x:Null}" ScrollViewer.VerticalScrollBarVisibility="Auto" ScrollViewer.HorizontalScrollBarVisibility="Auto" AllowDrop="True" VirtualizingStackPanel.IsVirtualizing="True" ScrollViewer.CanContentScroll="True" SelectionMode="Single"  IsSynchronizedWithCurrentItem="True" ItemContainerStyle="{DynamicResource RestaurantEditListViewItemStyle}" Style="{DynamicResource RestaurantEditListViewStyle}" IsEnabled="True" HorizontalAlignment="Stretch" VerticalAlignment="Top" x:Name="gvOrderDetails" >

<myClasses:SortableListView.View>

<GridView AllowsColumnReorder="False" >

<GridViewColumn Width="4"  HeaderContainerStyle="{DynamicResource FirstColumnGridViewColumnHeader}" CellTemplate="{StaticResource EmptyColumnTemplate}"/>

 <GridViewColumn Width="8" CellTemplate="{StaticResource NotEditableStateColumnTemplate}" HeaderContainerStyle="{DynamicResource RestaurantDataGridViewColumnHeader}"/>

<myClasses:SortListViewColumn Header="??? ?????" SortProperty="ProductBarcode" 

Width="80" CellTemplate="{StaticResource ProductBarcodeColumnTemplate}"                                         SortStyle="RestaurantDataGridViewColumnHeader" HeaderContainerStyle="{DynamicResource RestaurantDataGridViewColumnHeader}" />

<myClasses:SortListViewColumn Header="???? ????????" SortProperty="ProductName" 

Width="200" CellTemplate="{StaticResource ProdNameColumnTemplate}" SortStyle="RestaurantDataGridViewColumnHeader" HeaderContainerStyle="{DynamicResource RestaurantDataGridViewColumnHeader}" />

<myClasses:SortListViewColumn Header="?????"  SortProperty="SalePrice" CellTemplate="{StaticResource PriceNameColumnTemplate}" Width="60" SortStyle="RestaurantDataGridViewColumnHeader" HeaderContainerStyle="{DynamicResource RestaurantDataGridViewColumnHeader}" />

<myClasses:SortListViewColumn Header="??????"  SortProperty="UnitName" Width="50" 

CellTemplate="{StaticResource QuntityColumnTemplate}" SortStyle="RestaurantDataGridViewColumnHeader" HeaderContainerStyle="{DynamicResource RestaurantDataGridViewColumnHeader}"/>

<GridViewColumn Width="4" CellTemplate="{StaticResource EmptyColumnTemplate}" 

HeaderContainerStyle="{DynamicResource LastColumnGridViewColumnHeader}"/>

</GridView>

</myClasses:SortableListView.View>

</myClasses:SortableListView>

Now I want to get the total sum (which is sum(SalePrice*SoldQuantity) to display in textbox, And SoldQuantity value can changed by user