ASP.Net FromView with calculated textbox

I created an ASP formview for my SharePoint server with SharePoint Designer actually using the update template which contain some of textboxes, reference to below code I need help to make the following calculations:  Cost_CustomerTextBox = (QuantityTextBox * CostItem_1TextBox) + CostItem_2TextBox + CostItem_3TextBox + CostItem_4TextBox + CostItem_5TextBox + CostItem_6TextBox + CostItem_7TextBox + CostItem_8TextBox + CostItem_9TextBox
Source Code:
<asp:SqlDataSource runat="server" id="Customer" ProviderName="System.Data.SqlClient" ConnectionString="Data Source=maadi04;Initial Catalog=Visitors;Persist Security Info=True;User ID=sa;Password=sa" SelectCommand="SELECT [C_id], [Cust_nam] FROM [Trans_Customer] order by [Cust_nam]"> </asp:SqlDataSource> <asp:SqlDataSource runat="server" id="Forwarder" ProviderName="Sys ...