A way to control MarketPress product prices so they don't get set to <= 0

mp-price-control.php

<?php
/*
Plugin Name: MarletPress Control Product Prices
Plugin URI: https://premium.wpmudev.org/
Description: A way to control MarketPress product prices so they don't get set to <= 0
Author: Panos Lyrakis @ WPMUDEV
Author URI: https://premium.wpmudev.org/
License: GPLv2 or later
*/
if( ! class_exists( 'WPMUDEV_MP_Price_Control' ) ){

	class WPMUDEV_MP_Price_Control{

		private static $_instance = null;
		
		public static function get_instance() {
			if ( is_null( self::$_instance ) ) {
				self::$_instance = new WPMUDEV_MP_Price_Control();
			}
			return self::$_instance;
		}

		private function __construct(){

			add_action( "updated_post_meta", array( $this, 'updated_product_meta' ), 999, 4 );

		}

		public function updated_product_meta( $meta_id, $object_id, $meta_key, $_meta_value ){
			
			$post = get_post( $object_id );

			if( $post->post_type != MP_Product::get_post_type() || $meta_key != 'regular_price' || $_meta_value == '' ){
				return;
			}

			if( $_meta_value <= 0 ){

				$previous_valid_price = $this->get_previous_valid_price( $object_id );
				$this->force_product_price( $meta_id, $object_id, $meta_key, $previous_valid_price );

			}
			else{

				$meta_key = '_wpmudev_mp_regular_price_backup';
				update_post_meta( $object_id, '_wpmudev_mp_regular_price_backup', $_meta_value );

			}
			
		}

		public function force_product_price( $meta_id, $object_id, $meta_key, $_meta_value ){

			$meta_type = 'post';

			if ( !$meta_key || ! is_numeric( $_meta_value ) )
	                return false;
	
	        if ( !$object_id = absint($object_id) )
	                return false;
	
	        if ( ! $table = _get_meta_table($meta_type) )
	                return false;
	
	        global $wpdb;
	
	        $column = esc_sql($meta_type . '_id');
	        $id_column = 'user' == $meta_type ? 'umeta_id' : 'meta_id';
	
	        // expected_slashed ($meta_key)
	        $meta_key = stripslashes($meta_key);
	
	        $data  = array(
	        	'meta_value' => $_meta_value
	        );//compact( 'meta_value' );

	        $where = array( $column => $object_id, 'meta_key' => $meta_key );

	        $wpdb->update( $table, $data, $where );
	        wp_cache_delete($object_id, $meta_type . '_meta');

		}


		public function get_previous_valid_price( $post_id ){
			
			return get_post_meta( $post_id, '_wpmudev_mp_regular_price_backup', true );

		}

	}

	add_action( 'plugins_loaded', function(){
        $GLOBALS['WPMUDEV_MP_Price_Control'] = WPMUDEV_MP_Price_Control::get_instance();
    }, 10 );

}