How to group my Data Grid by modules?

Example:

Obs:

XAML File - Suggested Layout

Note: I declare here the xmlns: local="clr-namespace: WpfApplication1", so you can enjoy in <Window.Resources><local:TotalSum x:Key="TotalSum" /></Window.Resources> , where TotalSum is the class responsible for adding the items of each group. This item will be used in: <TextBlock Grid.Row="0" Grid.Column="1" HorizontalAlignment="Left" Text="{Binding Path=Items, Converter={StaticResource TotalSum}}" /> .

<Window x:Class="WpfApplication1.MainWindow" xmlns:local="clr-namespace:WpfApplication1"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525" Activated="Window_Activated" Initialized="Window_Initialized">
    <Window.Resources>
        <local:TotalSum  x:Key="TotalSum" />        
    </Window.Resources>
    <Grid>
        <DataGrid x:Name="GridVendas" CanUserAddRows="False" HorizontalAlignment="Left" Margin="10,10,0,0" VerticalAlignment="Top" Height="299" Width="497">            
            <DataGrid.GroupStyle>
                <GroupStyle>
                    <!--Header Cabeçalho -->
                    <GroupStyle.HeaderTemplate>
                        <DataTemplate>
                            <StackPanel>
                                <TextBlock Text="{Binding Path=Name}" />
                            </StackPanel>
                        </DataTemplate>
                    </GroupStyle.HeaderTemplate>
                    <!--Container Linhas -->
                    <GroupStyle.ContainerStyle>
                        <Style TargetType="{x:Type GroupItem}">
                            <Setter Property="Template">
                                <Setter.Value>
                                    <ControlTemplate TargetType="{x:Type GroupItem}">
                                        <DockPanel>
                                            <Grid DockPanel.Dock="Bottom">                                                
                                                <Grid.ColumnDefinitions>
                                                    <ColumnDefinition Width="*" />
                                                    <ColumnDefinition Width="*" />
                                                </Grid.ColumnDefinitions>
                                                <Grid.RowDefinitions>
                                                    <RowDefinition />
                                                    <RowDefinition />
                                                </Grid.RowDefinitions>
                                                <StackPanel Orientation="Horizontal">
                                                    <TextBlock Grid.Column="0" Text="{Binding Path=Name}" />
                                                    <TextBlock Grid.Column="0" Text=" R$: " />
                                                    <TextBlock Grid.Row="0" Grid.Column="1" HorizontalAlignment="Left" Text="{Binding Path=Items, Converter={StaticResource TotalSum}}" />
                                                </StackPanel>                                                                                                
                                            </Grid>
                                            <ItemsPresenter />
                                        </DockPanel>
                                    </ControlTemplate>
                                </Setter.Value>
                            </Setter>
                        </Style>
                    </GroupStyle.ContainerStyle>
                </GroupStyle>
            </DataGrid.GroupStyle>
        </DataGrid>
    </Grid>
</Window>

Model Classes - Will be responsible for WPF DataGrid Columns

public class ItemVendas
{
    public String Nome { get; set; }
    public Decimal Valor { get; set; }
    public String Grupo { get;set;}
}

TotalSum Class - Implementing Class of IValueConverter and will be responsible for adding the elements of each group

public class TotalSum: IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        decimal sum = 0;
        foreach (object item in (System.Collections.ObjectModel.ReadOnlyObservableCollection<object>)value)
        {
            ItemVendas it = (ItemVendas)item;
            sum += it.Valor;
        }
        return sum;
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

Loading - Window_Initialized Method - Examples are given at suggestion

Notice that you used ListCollectionView , the collection class you set up the grouping by property Grupo .

private void Window_Initialized(object sender, EventArgs e)
{

    ListItemVendas = new ObservableCollection<ItemVendas>();
    ListItemVendas.Add(new ItemVendas() { Grupo = "Vendas à dinheiro", Nome = "Suco de Uva", Valor = 2.50M });
    ListItemVendas.Add(new ItemVendas() { Grupo = "Vendas à dinheiro", Nome = "Pão de Queijo", Valor = 2.00M });

    ListItemVendas.Add(new ItemVendas() { Grupo = "Vendas à Cartão", Nome = "Suco de Laranja", Valor = 3.50M });
    ListItemVendas.Add(new ItemVendas() { Grupo = "Vendas à Cartão", Nome = "Quible", Valor = 4.00M });
    ListItemVendas.Add(new ItemVendas() { Grupo = "Vendas à Cartão", Nome = "Suco Copo", Valor = 1.50M });

    var listaView = new ListCollectionView(ListItemVendas);
    listaView.GroupDescriptions.Add(new PropertyGroupDescription("Grupo"));            

    GridVendas.AutoGenerateColumns = true;            
    GridVendas.ItemsSource = listaView;    

}

Final Score