Hi,

Check the below code!

<div>

    <asp:gridview ID="Gridview1" runat="server" EnableViewState="true"  ShowFooter="true"
                             AutoGenerateColumns="false">
        <Columns>
        <asp:BoundField DataField="RowNumber" HeaderText="Row Number" />
        <asp:TemplateField HeaderText="Header 1">
            <ItemTemplate>
                <asp:TextBox ID="Rate" runat="server" Text='<%#Eval("Column1") %>'   CssClass="textSelector"></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Header 2">
            <ItemTemplate>
                <asp:TextBox ID="Quantity" runat="server" Text='<%#Eval("Column2") %>'   CssClass="textSelector"></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Header 3">
            <ItemTemplate>
                 <asp:Label ID="Amount" runat="server" Text='<%#Eval("Column3") %>'  ></asp:Label>
            </ItemTemplate>
            <FooterStyle HorizontalAlign="Right" />
            <FooterTemplate>
            Total<asp:Label ID="AmountTotal" runat="server"></asp:Label><br />
            Tax<asp:Textbox ID="Tax" runat="server"></asp:Textbox><br />
            Grand Total<asp:Label ID="GT" runat="server"></asp:Label><br />
            <asp:Button ID="ButtonAdd" runat="server" Text="Add New Row" OnClick="ButtonAdd_Click1" />
            </FooterTemplate>
        </asp:TemplateField>
        </Columns>
</asp:gridview>

    <asp:HiddenField runat="server" ID="hAmountValue" />
    
    </div>

     <script type="text/javascript">
         $(document).ready(function () {
             $(".textSelector").keyup(function () {
                 var row = $(this).closest("tr");
                 var Rate = parseFloat(row.find("input[type=text][id*=Rate]").val());
                 if (isNaN(Rate)) Rate = 0;
                 var Quantity = parseFloat(row.find("input[type=text][id*=Quantity]").val());
                 if (isNaN(Quantity)) Quantity = 0;
                 row.find("span[id*=Amount]").html(Rate * Quantity);

                 document.getElementById('hAmountValue').value = Rate * Quantity;

                 var total = calculateTotal();
                 var taxValue = parseFloat($("span[id*=TaxValue]").html());
                 if (isNaN(taxValue)) taxValue = 0;
                 $("span[id*=GrandtotalWithTax]").val(total + taxValue);
             });
         });
         function calculateTotal() {
             var total = 0;
             $("span[id*=Amount]").each(function () {
                 temp = parseFloat($(this).html());
                 if (isNaN(temp)) temp = 0;
                 total = total + temp;
             });
             $("span[id*=GrandTotalWithoutTax]").html(total);
             return total;
         }

    </script>

Check the code behind!

  protected void Page_Load(object sender, EventArgs e)
        {
            if (!Page.IsPostBack)
            {
            
                SetInitialRow();

            } 

        }

        private void SetInitialRow()
        {

            DataTable dt = new DataTable();
            DataRow dr = null;
            dt.Columns.Add(new DataColumn("RowNumber", typeof(string)));
            dt.Columns.Add(new DataColumn("Column1", typeof(string)));
            dt.Columns.Add(new DataColumn("Column2", typeof(string)));
            dt.Columns.Add(new DataColumn("Column3", typeof(string)));
            dt.Columns.Add(new DataColumn("AmountTotal", typeof(string)));
            dt.Columns.Add(new DataColumn("Tax", typeof(string)));
            dt.Columns.Add(new DataColumn("GT", typeof(string)));

            dr = dt.NewRow();
            dr["RowNumber"] = 1;
            dr["Column1"] = string.Empty;
            dr["Column2"] = string.Empty;
            dr["Column3"] = string.Empty;
            dr["AmountTotal"] = string.Empty;
            dr["Tax"] = string.Empty;
            dr["GT"] = string.Empty;
            dt.Rows.Add(dr);

            //Store the DataTable in Session 
            Session["CurrentTable"] = dt;

            Gridview1.DataSource = dt;
            Gridview1.DataBind();
        }



        private void AddNewRowToGrid()
        {
            int nLastRow = Gridview1.Rows.Count - 1;

            TextBox txtRate = Gridview1.Rows[nLastRow].Cells[0].FindControl("Rate") as TextBox;
            TextBox txtQuantity = Gridview1.Rows[nLastRow].Cells[0].FindControl("Quantity") as TextBox;
            //Label txtAmount = gridView.Rows[nLastRow].Cells[0].FindControl("Amount") as Label;

            DataTable dataTable = Session["CurrentTable"] as DataTable;

            DataRow myRow = dataTable.Rows[dataTable.Rows.Count - 1];
            myRow["Column1"] = txtRate.Text;
            myRow["Column2"] = txtQuantity.Text;

            //Since the Label content can't be retrieved, it will be stored in the 
            //hidden variable and retr
            myRow["Column3"] = hAmountValue.Value;
            
            //Add Rest of the columns

            DataRow dataRow = null;

            dataRow = dataTable.NewRow();
            dataRow["RowNumber"] = 1;
            dataRow["Column1"] = "";
            dataRow["Column2"] = "";
            dataRow["Column3"] = "";

            //Add Rest of the columns


            dataTable.Rows.Add(dataRow);

            Gridview1.DataSource = dataTable;
            Gridview1.DataBind();

            Session["CurrentTable"] = dataTable;
        }

        protected void ButtonAdd_Click1(object sender, EventArgs e)
        {
            AddNewRowToGrid();
        } 

Hope it helps u...