how to sum textbox value and dropdown value in asp.net..
the dropdown value as 300
and the textbox value is 30
total is 330
my design is
<table class="style2" align="center" style="border-color: #1d599e; border-style: outset;
border-width: thin; background-color: #D7D7D7; width:50%" >
<tr>
<td style="margin-left: 80px; background:#1d599e;" align="center" colspan="2">
BILLING DETAILS
</td>
</tr>
<tr>
<td colspan="2">
</td>
</tr>
<tr align="left">
<td>
<asp:Label ID="Label1" runat="server" Text="Product" Visible="false"></asp:Label>
</td>
<td>
<asp:TextBox ID="TextBox2" runat="server" Visible="false" Text="Cement"></asp:TextBox>
</td>
</tr>
<tr align="left">
<td>
<asp:Label ID="Label2" runat="server" Text="Date"></asp:Label>
</td>
<td>
<asp:TextBox ID="txtdate" runat="server"></asp:TextBox>
</td>
</tr>
<tr align="left">
<td>
<asp:Label ID="Label3" runat="server" Text="BillNo"></asp:Label>
</td>
<td>
<asp:TextBox ID="txtbillno" runat="server"
></asp:TextBox>
</td>
</tr>
<tr align="left">
<td>
<asp:Label ID="Label4" runat="server" Text="Customer Name"></asp:Label>
</td>
<td>
<asp:TextBox ID="txtcn" runat="server" ></asp:TextBox>
</td>
</tr>
<tr align="left">
<td>
<asp:Label ID="Label5" runat="server" Text="Address"></asp:Label>
</td>
<td>
<asp:TextBox ID="txtad" runat="server"></asp:TextBox>
</td>
</tr>
<tr align="left">
<td>
<asp:Label ID="Label6" runat="server" Text="Brand Name"></asp:Label> </td>
<td>
<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True"
onselectedindexchanged="DropDownList1_SelectedIndexChanged" Width="146px">
</asp:DropDownList>
</td>
</tr>
<tr align="left">
<td>
<asp:Label ID="Label7" runat="server" Text="Quantity"></asp:Label> </td>
<td>
<asp:DropDownList ID="DropDownList2" runat="server" AutoPostBack="True"
onselectedindexchanged="DropDownList2_SelectedIndexChanged" Height="16px"
Width="146px">
<asp:ListItem>Select</asp:ListItem>
</asp:DropDownList>
</td>
</tr>
<tr align="left">
<td>
<asp:Label ID="Label8" runat="server" Text="Price"></asp:Label>
</td>
<td>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
</td>
</tr>
<tr>
<td>
<asp:Label ID="Label11" runat="server" Text="Required No"></asp:Label>
</td>
<td>
<asp:DropDownList ID="DropDownList3" runat="server"
onselectedindexchanged="DropDownList3_SelectedIndexChanged" Width="149px">
</asp:DropDownList>
<asp:Label ID="Label14" runat="server" Visible="False"></asp:Label>
</td>
</tr>
<tr>
<td>
<asp:Label ID="Label12" runat="server" Text="TotalPrice"></asp:Label>
</td>
<td>
<asp:TextBox ID="txttp" runat="server"></asp:TextBox>
</td>
</tr>
<tr>
<td>
<asp:Label ID="Label13" runat="server" Text="Vat"></asp:Label>
</td>
<td>
<asp:TextBox ID="txtvat" runat="server"></asp:TextBox>
</td>
</tr>
<tr>
<td>
vat/</td>
<td>
<asp:TextBox ID="TextBox3" runat="server"></asp:TextBox>
</td>
</tr>
<tr align="left">
<td>
<asp:Label ID="Label9" runat="server" Text="Extra"></asp:Label>
</td>
<td>
<asp:TextBox ID="txtex" runat="server"></asp:TextBox>
</td>
</tr>
<tr>
<td colspan="2" align="center">
<asp:Button ID="Button1" runat="server" Text="Submit" onclick="Button1_Click" />
<asp:Label ID="Label10" runat="server" Text=""></asp:Label>
<asp:Button ID="Button2" runat="server" Text="clear" onclick="Button2_Click" />
</td>
</tr>
</table>
i need requiredno * price