EDD – Free download

$meta_query = array(
	'relation' => 'AND',
	array(
		'relation' => 'OR',
		array(
			'key'     => 'edd_price',
			'compare' => 'NOT EXISTS',
		),
		array(
			'key'     => 'edd_price',
			'value'   => 'zero',
		),
	),
	array(
		'relation' => 'OR',
		array(
			'key'     => 'edd_variable_prices',
			'compare' => 'NOT EXISTS',
		),
		array(
			'key'     => 'edd_variable_prices',
			'value'   => 'a:0:{}',
		),
	),
);