<asp:Button ID="T1801" CssClass="costlinks" runat="server" Text='<%# T1801.ClientID %>' />
or
<asp:Button ID="T1801" CssClass="costlinks" runat="server" Text='<%# string.Format("The Client ID is {0}", T1801.ClientID) %>' />
or if you still want to use code-behind
<asp:Button ID="T1801" CssClass="costlinks" runat="server" Text='<%# FF_1(T1801) %>' />
protected string FF_1(Button button)
{
return button.ClientID;
}