WinForms Data Grid - How to calculate a total summary based on other columns' summary values

C#
using DevExpress.XtraGrid;
using DevExpress.XtraGrid.Views.Base;
using DevExpress.XtraGrid.Views.Grid;
using System;
using System.Data;
using System.Windows.Forms;

namespace Q273845
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            productsBindingSource.DataSource = GetProductsDataTable();
        }

        DataTable GetProductsDataTable()
        {
            DataTable table = new DataTable();
            table.TableName = "Products";
            table.Columns.Add(new DataColumn("ProductName", typeof(string)));
            table.Columns.Add(new DataColumn("UnitsInStock", typeof(int)));
            table.Columns.Add(new DataColumn("UnitPrice", typeof(double)));
            table.Columns.Add(new DataColumn("Category", typeof(string)));
            Random random = new Random();
            for(int i = 0; i < 20; i++)
            {
                int index = i + 1;
                table.Rows.Add("Product " + index, random.Next(1, 20), Math.Round(random.NextDouble() * 1000, 3), "Category " + i % 7);
            }
            return table;
        }

        private void gridView1_CustomDrawFooterCell(object sender, FooterCellCustomDrawEventArgs e)
        {
            if(e.Column == colPrice)
                e.Info.DisplayText = (Convert.ToDecimal(colUnitPrice.SummaryItem.SummaryValue) *
                    Convert.ToDecimal(colUnitsInStock.SummaryItem.SummaryValue)).ToString();
        }

        private void gridView1_CustomDrawRowFooterCell(object sender, FooterCellCustomDrawEventArgs e)
        {
            if(e.Column == colPrice)
            {
                GridView view = (GridView)sender;
                e.Info.DisplayText = (Convert.ToDecimal(view.GetGroupSummaryValue(e.RowHandle,
                    (GridGroupSummaryItem)view.GroupSummary["UnitPrice"])) *
                    Convert.ToDecimal(view.GetGroupSummaryValue(e.RowHandle,
                    (GridGroupSummaryItem)view.GroupSummary["UnitsInStock"]))).ToString();
            }
        }

        private void gridView1_CustomUnboundColumnData(object sender, CustomColumnDataEventArgs e)
        {
            GridView view = (GridView)sender;
            if(e.Column == colPrice)
                if(e.IsGetData)
                    e.Value = Convert.ToDecimal(view.GetListSourceRowCellValue(e.ListSourceRowIndex, colUnitPrice)) *
                        Convert.ToInt32(view.GetListSourceRowCellValue(e.ListSourceRowIndex, colUnitsInStock));
        }
    }
}

Visual Basic
Imports DevExpress.XtraGrid
Imports DevExpress.XtraGrid.Views.Base
Imports DevExpress.XtraGrid.Views.Grid
Imports System
Imports System.Data
Imports System.Windows.Forms

Namespace Q273845
    Partial Public Class Form1
        Inherits Form

        Public Sub New()
            InitializeComponent()
        End Sub

        Private Sub Form1_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
            productsBindingSource.DataSource = GetProductsDataTable()
        End Sub

        Private Function GetProductsDataTable() As DataTable
            Dim table As DataTable = New DataTable()
            table.TableName = "Products"
            table.Columns.Add(New DataColumn("ProductName", GetType(String)))
            table.Columns.Add(New DataColumn("UnitsInStock", GetType(Integer)))
            table.Columns.Add(New DataColumn("UnitPrice", GetType(Double)))
            table.Columns.Add(New DataColumn("Category", GetType(String)))
            Dim random As Random = New Random()
            For i As Integer = 0 To 20 - 1
                Dim index As Integer = i + 1
                table.Rows.Add("Product " & index, random.[Next](1, 20), Math.Round(random.NextDouble() * 1000, 3), "Category " & i Mod 7)
            Next
            Return table
        End Function

        Private Sub gridView1_CustomDrawFooterCell(ByVal sender As Object, ByVal e As FooterCellCustomDrawEventArgs) Handles gridView1.CustomDrawFooterCell
            If e.Column Is colPrice Then e.Info.DisplayText = (Convert.ToDecimal(colUnitPrice.SummaryItem.SummaryValue) * Convert.ToDecimal(colUnitsInStock.SummaryItem.SummaryValue)).ToString()
        End Sub

        Private Sub gridView1_CustomDrawRowFooterCell(ByVal sender As Object, ByVal e As FooterCellCustomDrawEventArgs) Handles gridView1.CustomDrawRowFooterCell
            If e.Column Is colPrice Then
                Dim view As GridView = CType(sender, GridView)
                e.Info.DisplayText = (Convert.ToDecimal(view.GetGroupSummaryValue(e.RowHandle, CType(view.GroupSummary("UnitPrice"), GridGroupSummaryItem))) * Convert.ToDecimal(view.GetGroupSummaryValue(e.RowHandle, CType(view.GroupSummary("UnitsInStock"), GridGroupSummaryItem)))).ToString()
            End If
        End Sub

        Private Sub gridView1_CustomUnboundColumnData(ByVal sender As Object, ByVal e As CustomColumnDataEventArgs) Handles gridView1.CustomUnboundColumnData
            Dim view As GridView = CType(sender, GridView)
            If e.Column Is colPrice Then
                If e.IsGetData Then e.Value = Convert.ToDecimal(view.GetListSourceRowCellValue(e.ListSourceRowIndex, colUnitPrice)) * Convert.ToInt32(view.GetListSourceRowCellValue(e.ListSourceRowIndex, colUnitsInStock))
            End If
        End Sub
    End Class
End Namespace