WPF: Binde items control inde i items control.

<Window x:Class="WpfCollectionX2.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1" Height="300" Width="300"
    DataContext="{Binding RelativeSource={RelativeSource Self}}">
    <Window.Resources>
        <DataTemplate x:Key="Lines">
            <StackPanel Orientation="Horizontal">
                <TextBlock Text="{Binding Description}" Width="120" ></TextBlock>
                <TextBlock Text="{Binding Amount}"  TextAlignment="Right" Width="120" ></TextBlock>
            </StackPanel>
        </DataTemplate>
        <DataTemplate x:Key="Groups">
                <GroupBox Header="{Binding GroupName}">
                    <ItemsControl ItemsSource="{Binding BudgetLines}" ItemTemplate="{StaticResource Lines}">
                    </ItemsControl>
                </GroupBox>
        </DataTemplate>
    </Window.Resources>

    <Grid>
            <ItemsControl ItemsSource="{Binding BudgetGroups}" ItemTemplate="{StaticResource Groups}">
            </ItemsControl>
    </Grid>
</Window>

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Collections.ObjectModel;

namespace WpfCollectionX2
{
    public partial class Window1 : Window
    {
        ObservableCollection<BudgetGroup> budgetGroups = new ObservableCollection<BudgetGroup>();
        public ObservableCollection<BudgetGroup> BudgetGroups { get { return budgetGroups; } }

        public Window1()
        {
            budgetGroups.Add(new BudgetGroup() { GroupName = "Test01" });
            budgetGroups[0].addBudgetLine(new BudgetLine() { Description = "Line 01 01", Amount = 101m });
            budgetGroups[0].addBudgetLine(new BudgetLine() { Description = "Line 01 02", Amount = 102m });
            budgetGroups.Add(new BudgetGroup() { GroupName = "Test02" });
            budgetGroups[1].addBudgetLine(new BudgetLine() { Description = "Line 02 01", Amount = 201m });
            budgetGroups[1].addBudgetLine(new BudgetLine() { Description = "Line 02 02", Amount = 202m });
            budgetGroups[1].addBudgetLine(new BudgetLine() { Description = "Line 02 03", Amount = 203m });

            InitializeComponent();
        }
    }

    public sealed class BudgetGroup
    {
        ObservableCollection<BudgetLine> budgetLines = new ObservableCollection<BudgetLine>();

        public string GroupName { get; set; }
        public ObservableCollection<BudgetLine> BudgetLines { get { return budgetLines; } }
       
        public void addBudgetLine(BudgetLine line) {
            this.budgetLines.Add(line);
        }
    }
    public sealed class BudgetLine
    {
        public string Description { get; set; }
        public decimal Amount { get; set; }
    }

}