Order by meta_value

gistfile1.txt

$args = array(
	'showposts' => 20,
	'post_type' => array( 'automoviles' ),
	'meta_key' => 'Precio',
	'orderby' => 'meta_value_num',
	'order'  => 'ASC',
	'post_parent' => $postid,
	'tax_query' => array(
		'relation' => 'AND',
		array(
			'taxonomy' => 'marcas',
			'field' => 'slug',
			'terms' => array( $marcaslug )
		)
	)
);