How can I programmatically add triggers to an ASP.NET UpdatePanel?

Question

I am trying to write a quote generator. For each product, there are a set of options. I want to dynamically add a drop down list for each option, and then have their SelectedIndexChanged events all wired up to update the quote cost.

I am not having any trouble adding the DropDownList controls to my UpdatePanel, but I can't seem to wire up the events.

After the page loads, the drop downs are there, with their data, but changing them does not call the SelectedIndexChanged event handler, nor does the QuoteUpdatePanel update. I have something like this:

Edit: Since programmatically adding AsyncPostBackTrigger controls is not supported, I've change my code to this, but I still don't get the event:

Edit 2: Tried adding a PlaceHolder to add the drop down lists to (instead directly into the ContentTemplateContainer, still no events firing.

QuotePanel.ASCX

<asp:ScriptManager ID="ScriptManager" runat="server" />

<asp:UpdatePanel ID="QuoteUpdatePanel" runat="server" ChildrenAsTriggers="true">
    <ContentTemplate>
        Cost: <asp:Label ID="QuoteCostLabel" runat="server" />
        <fieldset id="standard-options">
            <legend>Standard Options</legend>
            <asp:UpdatePanel ID="StandardOptionsUpdatePanel" runat="server" ChildrenAsTriggers="true" UpdateMode="Conditional">
                <ContentTemplate>
                <asp:PlaceHolder ID="StandardOptionsPlaceHolder" runat="server" />                   
                </ContentTemplate>
            </asp:UpdatePanel>
        </fieldset>
    </ContentTemplate>
</asp:UpdatePanel>

The code to add the dropdowns and the event they are to be wire up for:

protected void PopluateUpdatePanel(IEnumerable<IQuoteProperty> standardOptions)
{
    foreach (IQuoteProperty standardOp in standardOptions)
    {
        QuotePropertyDropDownList<IQuoteProperty> dropDownList = new QuotePropertyDropDownList<IQuoteProperty>(standardOp);
        dropDownList.SelectedIndexChanged += QuotePropertyDropDown_SelectedIndexChanged;
        dropDownList.ID = standardOp.GetType().Name + "DropDownList";
        dropDownList.CssClass = "quote-property-dropdownlist";

        Label propertyLabel = new Label() {Text = standardOp.Title, CssClass = "quote-property-label"};

        StandardOptionsPlaceHolder.Controls.Add(propertyLabel);
        StandardOptionsPlaceHolder.Controls.Add(dropDownList);

        _standardOptionsDropDownLists.Add(dropDownList);

        ScriptManager.RegisterAsyncPostBackControl(dropDownList);

    }

}

void QuotePropertyDropDown_SelectedIndexChanged(object sender, EventArgs e)
{
    QuoteCostLabel.Text = QuoteCost.ToString();
    StandardOptionsUpdatePanel.Update();
}