Lost Focus on textbox in windows store apps

Hello EveryBody!

I need to get value of the textbox when it lost focus in the Grid.Column="9"

Here is my XAML code on Windows Store apps, I'm using MVVM:

                  <ItemsControl ItemsSource="{Binding Order.Products, Mode=TwoWay}"
                                x:Name="ItemsControlOrderProducts">
                    <ItemsControl.ItemTemplate>
                      <DataTemplate>
                        <Grid  Height="60"
                               Margin="0 0 0 12"
                               Background="{StaticResource GrayLightSolidColorBrush}">
                          <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="15" />
                            <ColumnDefinition Width="Auto" />
                            <ColumnDefinition Width="25" />
                            <ColumnDefinition Width="140" />
                            <ColumnDefinition Width="40" />
                            <ColumnDefinition Width="57" />
                            <ColumnDefinition Width="110" />
                            <ColumnDefinition Width="57" />
                            <ColumnDefinition Width="85" />
                            <ColumnDefinition Width="130" />
                            <ColumnDefinition Width="80" />
                            <ColumnDefinition Width="80" />
                            <ColumnDefinition Width="15" />
                          </Grid.ColumnDefinitions>
                          <Grid Grid.Column="1"
                                Background="Transparent"
                                Margin="0 0 -12 0">
                            <controls:TkButton  Template="{StaticResource DeleteIcon}"
                                                Width="15"
                                                HorizontalAlignment="Center"
                                                Background="Transparent"
                                                VerticalAlignment="Top"
                                                Margin="-2 6 0 0"
                                                Command="{Binding DataContext.RemoveOrderProductCommand, ElementName=ItemsControlOrderProducts}"
                                                CommandParameter="{Binding}" />
                          </Grid>
                          <StackPanel Grid.Column="3"
                                      HorizontalAlignment="Center"
                                      VerticalAlignment="Center">
                            <TextBlock Text="{Binding Product.Label}"
                                       Style="{StaticResource BlueDarkLight16TextBlockStyle}"
                                       Width="140"
                                       TextTrimming="CharacterEllipsis" />
                            <TextBlock Text="{Binding Product.UnderProduct}"
                                       Style="{StaticResource DarkLightLight16TextBlockStyle}"
                                       Margin="10 0 0 0"
                                       Width="130"
                                       TextTrimming="CharacterEllipsis" />
                          </StackPanel>
                          <TextBlock Grid.Column="5"
                                     Text="{Binding Price}"
                                     Visibility="{Binding Price, Converter={StaticResource PricelistVisibilityConverter}}"
                                     HorizontalAlignment="Center"
                                     VerticalAlignment="Center"
                                     Style="{StaticResource DarkLightLight16TextBlockStyle}" />
                          <TextBlock Grid.Column="7"
                                     Text="{Binding Balance}"
                                     HorizontalAlignment="Center"
                                     VerticalAlignment="Center"
                                     Style="{StaticResource DarkLightLight16TextBlockStyle}" />
                          <TextBox Grid.Column="9"
                                   Text="{Binding Quantity, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay, Converter={StaticResource DecimalToStringConverter} }"
                                   HorizontalAlignment="Center"
                                   VerticalAlignment="Center"
                                   TextAlignment="Right"
                                   Width="70"
                                   FontFamily="Global User Interface">
                            <i:Interaction.Behaviors>
                              <core:EventTriggerBehavior EventName="LostFocus">
                                <core:InvokeCommandAction Command="{Binding DataContext.CalculateTotalCostCommand, ElementName=ItemsControlOrderProducts}"
                                                          CommandParameter="{Binding}" />
                              </core:EventTriggerBehavior>

                            </i:Interaction.Behaviors>
                          </TextBox>
                          <TextBlock Grid.Column="11"
                                     Text="{Binding TotalCost, UpdateSourceTrigger=PropertyChanged, Converter={StaticResource DecimalToStringConverter}, Mode=TwoWay}"
                                     HorizontalAlignment="Center"
                                     VerticalAlignment="Center"
                                     Foreground="{StaticResource BlueLightSolidColorBrush}"
                                     Style="{StaticResource DarkLightLight16TextBlockStyle}" />
                        </Grid>
                      </DataTemplate>
                    </ItemsControl.ItemTemplate>
                  </ItemsControl>

The problem is that it doesn't get the value of Text Property.

Someone have an idea?

Thanks