function event_list_donation_marker( $cost, $post_id ) {
	// Only add the donation marker in list view
	if ( ! tribe_is_list_view() ) return $cost;

	$fields = tribe_get_custom_fields( $post_id );

	if ( ! empty( $fields['Donation'] ) && 'Yes' === $fields['Donation'] )
		$cost .= ' BY DONATION ';

	return $cost;
}

add_filter( 'tribe_get_cost', 'event_list_donation_marker', 100, 2 );