sum asp unbound gridview column

Please I would like to get the sum or total of my total column values in the txtFootTotal text box. I can already multiply quantity and price and the value is shown in the total column so now i need to sum the total values and show the same in the textbox at the footer. Please note that my gridview textboxes are not calling back data from the database they are unbound.

Here is my gridview

 <asp:GridView ID="grvbill" runat="server"
                ShowFooter="True" AutoGenerateColumns="False"
                CellPadding="4" ForeColor="#333333"
                GridLines="None" >
    <Columns>
        <asp:BoundField DataField="RowNumber" HeaderText="SNo" />
        <asp:TemplateField HeaderText="quantity">
            <ItemTemplate>
                <asp:TextBox ID="qty" runat="server"></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="unit price">
            <ItemTemplate>
                <asp:TextBox ID="price" runat="server"></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="all total">
            <ItemTemplate>
                <asp:TextBox ID="total" runat="server"
                   Height="55px" TextMode="MultiLine"></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
       <asp:TemplateField HeaderText="">
            <ItemTemplate>

            </ItemTemplate>
            <FooterStyle HorizontalAlign="Right" />
            <FooterTemplate>
            <asp:TextBox ID="txtFootTotal" runat="server" Width="100px" Wrap="true" style="position:relative; right:500px;"></asp:TextBox>
                <asp:Button ID="ButtonAdd" runat="server"
                        Text="Add New Row" OnClick="ButtonAdd_Click" />
            </FooterTemplate>
        </asp:TemplateField>
           </Columns>
    <FooterStyle BackColor="" Font-Bold="True" ForeColor="" />
    <RowStyle BackColor="" />
    <EditRowStyle BackColor="" />
    <SelectedRowStyle BackColor="" Font-Bold="True" ForeColor="" />
    <PagerStyle BackColor="" ForeColor="" HorizontalAlign="Center" />
    <HeaderStyle BackColor="" Font-Bold="true" ForeColor="" />
    <AlternatingRowStyle BackColor="" />
</asp:GridView>


my javascript to do multiplication

<script type="text/javascript" language="javascript">
    function dowork(a, b, res) {
        document.getElementById(res).value = parseInt(document.getElementById(a).value, 10) * parseInt(document.getElementById(b).value, 10);
    }
</script>


my code behind in VB

Protected Sub grvbill_RowDataBound(ByVal sender As Object, ByVal e As GridViewRowEventArgs) Handles grvbill.RowDataBound

        If e.Row.RowType = DataControlRowType.DataRow Then
            Dim boxa As TextBox = TryCast(e.Row.FindControl("qty"), TextBox)
            Dim boxb As TextBox = TryCast(e.Row.FindControl("price"), TextBox)
            Dim boxc As TextBox = TryCast(e.Row.FindControl("total"), TextBox)

            boxb.Attributes.Add("onkeyup", "dowork('" + boxa.ClientID + "','" + boxb.ClientID + "','" + boxc.ClientID + "')")
        End If
    End Sub


Thanks.