How can I change the display of data in a TextBox from a DropDownList selection?

Hi guys, like the topic I posted. I really need to know how to get the output from the DDL that can automatically change the display in the TextBox. Each of our items is over one size and sometimes the price will differ when you click on a different size. How do you implement this feature?

Any help I would appreciate.

Here's my sample code:

Details.aspx:

<asp:TemplateField SortExpression="Size" ItemStyle-Wrap="True" ItemStyle-HorizontalAlign="Left" ItemStyle-VerticalAlign="Bottom" HeaderStyle-Wrap="False" HeaderText="Options">
  <ItemTemplate>
    Size:<asp:DropDownList ID="DropDownList1" runat="server" DataSourceID="SizeSource" AutoPostBack="true" DataTextField="Size" DataValueField="Size" Height="22px" Width="60px">
    </asp:DropDownList>
    &nbsp;&nbsp;&nbsp;&nbsp; Qty:<asp:TextBox ID="TextBox2" runat="server" Height="16px" Width="30px" Text="<%# 1 %>"></asp:TextBox>
    <br />
    Price(USD):
    <asp:TextBox runat="server" DataSourceID="SizeSource" SortExpression="Price" DataFormatString="{0}" ID="PriceList" Width="50px" Height="16px" ValidationGroup="Price" AutoPostBack="True" />
    <asp:SqlDataSource ID="SizeSource" runat="server" ConnectionString="<%$ ConnectionStrings:LIFEbabySAP %>" SelectCommand="SELECT left(T0.ItemCode,8) as Image, left(T0.ItemCode,8) as Code, T0.U_ItemName as Name, T0.U_Size as Size, T0.U_Colour as Color,case T1.Onhand when 0 then 'Out of Stock' else 'In Stock' end as Availability, T0.UserText as Description, CAST(T2.Price as Int)as Price FROM OITM T0 inner join oitw T1 on T0.ItemCode = T1.Itemcode inner join itm1 T2 on T0.ItemCode = T2.ItemCode WHERE (left(T0.ItemCode,8) = @Image) and T0.U_USE  = 'Y' and T1.Onhand != 0 and T1.whscode ='BAW' and T2.PriceList = 10">
      <SelectParameters>
        <asp:QueryStringParameter Name="Image" QueryStringField="product" Type="String" />
      </SelectParameters>
    </asp:SqlDataSource>
  </ItemTemplate>
  <HeaderStyle Wrap="False"></HeaderStyle>
  <ItemStyle HorizontalAlign="Left" VerticalAlign="Bottom" Wrap="True"></ItemStyle>
</asp:TemplateField>

      

        
        
        
      

    

Catalog.aspx.cs CodeBehind (C #):

private void Page_Load(object sender, System.EventArgs e)
{
  if (!IsPostBack)
  {
    string Size = Request.QueryString["Price"];
    if (Size != null)
    {
      ddlCategory.SelectedValue = Size;
    }
  }

  ItemsGet();

}

protected void CatalogList_SelectedIndexChanged(object sender, EventArgs e)
{
  string Image = CatalogList.SelectedValue.ToString().Trim();
  string catid = ddlCategory.SelectedValue;
  Response.Redirect("Details.aspx?product="
    + Image
    + "&Price="
    + catid);
}