ASP.Net gridview calculate with bind in templatefield

How come in asp.net that a calculated row doesn't multiply?

The code is given below, but this is not working:

Bind("aantal") * Bind("verkoopPrijs")

I want aantal (= ammount) x verkoopPrijs (= SalesPrice), but it only displays the salesPrice (verkoopPrijs).

Also, i want to know how a gridview total is set in a footertemplate (by adding the totals of 1 column, the column here is named "Bedrag")

<asp:GridView ID="gvWinkelwagentjeKoop" runat="server" AutoGenerateColumns="False" DataKeyNames="DVDId,soortAankoop">
                                        <Columns>
                                        <asp:BoundField DataField="naam" HeaderText="Categorie" ReadOnly="true" showHeader="false" />
                                        <asp:BoundField DataField="Titel" HeaderText="Titel" ReadOnly="true" showHeader="false" />
                                        <asp:BoundField DataField="verkoopPrijs" HeaderText="Prijs per eenheid" ReadOnly="true" showHeader="false" visible="true"/> 
                                        <asp:BoundField DataField="aantal" HeaderText="Aantal" ReadOnly="false" showHeader="false" /> 
                                            <asp:TemplateField HeaderText="Bedrag" ShowHeader="False">
                                                <EditItemTemplate>
                                                    <asp:TextBox ID="TextBox1" runat="server" Text='<%# Bind("aantal") * Bind("verkoopPrijs") %>'></asp:TextBox>
                                                </EditItemTemplate>
                                                <ItemTemplate>
                                                    <asp:Label ID="Label1" runat="server" Text='<%# Bind("aantal") * Bind("verkoopPrijs") %>'></asp:Label>
                                                </ItemTemplate>
                                            </asp:TemplateField>
                                            <asp:BoundField DataField="DVDId" HeaderText="DVD Id" ReadOnly="true" showHeader="false" visible="false"/>
                                            <asp:CommandField ShowDeleteButton="True" ShowEditButton="True" 
                                                CancelText="Annuleer" DeleteText="Verwijder" EditText="Aanpassen" 
                                                InsertText="Toevoegen" NewText="Nieuw" SelectText="Selecteer" 
                                                CausesValidation="False"/>
                                        </Columns>
                                        
                                    </asp:GridView></p><p>

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