SOLUTION: Figure out that form_select_options doesn't handle #default_value#default_value
at all. So I used #value property: $form['price']['min']['#value'] = isset($_REQUEST['price']['min']) ? $_REQUEST['price']['min'] :#value $my_default_value;property
:
$form['price']['min']['#value'] = isset($_REQUEST['price']['min']) ? $_REQUEST['price']['min'] : $my_default_value;