Query not sorting on meta_value field

Hi,

I have got a WP site with lots of real estate properties. I would like to order them on the price of the properties in descending order. The prices are stored in _property_price. However when I fire the query it displays them in the order in which I have published them. Someone can have a look at my php code?

Thanks

[ Moderator note: code fixed. Please wrap code in the backtick character without the blockquote. ]

$paged    = ( get_query_var( 'paged' ) ) ? get_query_var( 'paged' ) : 1;
		$posts_per_page = $tpl_default_settings['product-post-per-page'];
		$wp_query = new WP_Query();

		$properties = array(
			'post_type' =>'dt_properties',
			'posts_per_page' =>$post_per_page,
			'paged' =>$paged,
			'tax_query' =>array(),
			'meta_query' =>array(),
			'order' => 'DESC',
			'order_by' => 'meta_value_num', 'meta_key' => '_property_price');

		#Post Title
		if( isset($_REQUEST['pname']) ):

			# search_proprty_title - custom arg please refer property_title_filter()
			$properties['search_proprty_title'] = $_REQUEST['pname'];
			add_filter( 'posts_where', 'property_title_filter', 10, 2 );
		endif;

		#Contract Type
		if( isset($_REQUEST['searchby'])  && $_REQUEST['searchby'] !== "default" ):

            $contract_type_id = get_term_by('slug',$_REQUEST['searchby'],'contract_type',ARRAY_A);
            $contract_type_id = is_array( $contract_type_id ) ? $contract_type_id['term_id'] : "";

			$properties['tax_query'][] = array( 'taxonomy' => 'contract_type',
				'field' => 'id',
				'terms' => $contract_type_id,
				'operator' => 'IN',);
		endif;

		#Location
		if( !empty( $_REQUEST['plocation']) && $_REQUEST['plocation'] > 0 ) {
			$properties['tax_query'][] = array( 'taxonomy' => 'property_location',
				'field' => 'id',
				'terms' => $_REQUEST['plocation'],
				'operator' => 'IN',);
		}

		#Property Type
		if( !empty( $_REQUEST['ptype']) && $_REQUEST['ptype'] > 0 ) {
			$properties['tax_query'][] = array( 'taxonomy' => 'property_type',
				'field' => 'id',
				'terms' => $_REQUEST['ptype'],
				'operator' => 'IN',);
		}		

		# Beds Meta
		if( !empty( $_REQUEST['pbeds']) && $_REQUEST['pbeds'] > 0 ) {
			$properties['meta_query'][] = array(
				'key'     => '_bedrooms',
				'value'   => $_REQUEST['pbeds'],
				'compare' => '>=',
				'type'    => 'numeric',);
		}

		#Bath Meta
		if( !empty( $_REQUEST['pbaths']) && $_REQUEST['pbaths'] > 0 ) {
			$properties['meta_query'][] = array(
				'key'     => '_bathrooms',
				'value'   => $_REQUEST['pbaths'],
				'compare' => '>=',
				'type'    => 'numeric',);
		}

		#Floors Meta
		if( !empty( $_REQUEST['pfloors']) && $_REQUEST['pfloors'] > 0 ) {
			$properties['meta_query'][] = array(
				'key'     => '_floors',
				'value'   => $_REQUEST['pfloors'],
				'compare' => '>=',
				'type'    => 'numeric',);
		}

		#Parking Meta
		if( !empty( $_REQUEST['pgarages']) && $_REQUEST['pgarages'] > 0 ) {
			$properties['meta_query'][] = array(
				'key'     => '_parking',
				'value'   => $_REQUEST['pgarages'],
				'compare' => '>=',
				'type'    => 'numeric',);
		}

		#Area
		$minarea = isset($_REQUEST['minarea']) ? $_REQUEST['minarea'] : 0;
		if( $minarea > 0 ){

			$properties['meta_query'][] = array(
				'key'     => '_area',
				'value'   => $minarea,
				'type'    => 'numeric',
				'compare' => '>=');

		}		

		#Perceel
		$minperceel = isset($_REQUEST['minperceel']) ? $_REQUEST['minperceel'] : 0;
		if( $minperceel > 0 ){

			$properties['meta_query'][] = array(
				'key'     => '_perceel',
				'value'   => $minperceel,
				'type'    => 'numeric',
				'compare' => '>=');

		}				

		#Price
		$minprice = isset($_REQUEST['minprice']) ? $_REQUEST['minprice'] : 0;
		$maxprice = isset($_REQUEST['maxprice']) ? $_REQUEST['maxprice'] : 0;
		if( $minprice > 0 && $maxprice > 0 ){

			$properties['meta_query'][] = array(
				'key'     => '_property_price',
				'value'   => array( $minprice, $maxprice ),
				'type'    => 'numeric',
				'compare' => 'BETWEEN');

		}elseif( $minprice > 0 ){

			$properties['meta_query'][] = array(
				'key'     => '_property_price',
				'value'   => $minprice,
				'type'    => 'numeric',
				'compare' => '>=');

		}elseif( $maxprice > 0 ){

			$properties['meta_query'][] = array(
				'key'     => '_property_price',
				'value'   => $maxprice,
				'type'    => 'numeric',
				'compare' => '<=');
		}

		$wp_query->query( $properties );