Old code for WC 2.x to set a flash sale price on all products.

flashsale.php

<?php
// Exit if accessed directly
if ( ! defined( 'ABSPATH' ) ) {
	exit;
}
// Check if WP_CLI exists, and only extend it if it does
if ( defined( 'WP_CLI' ) && WP_CLI && ! class_exists( 'CP_CLI' ) ) {
	/**
	 * Class CP_CLI
	 */
	class CP_CLI extends WP_CLI_Command {
		public function flashsale( $args, $assoc_args ) {
			global $wpdb;
			$rate = isset( $assoc_args['rate'] ) ? absint( $assoc_args['rate'] ) : 20;
			$sale = absint( $rate ) / 100;
			$products = $wpdb->get_col( "select ID from {$wpdb->posts} where post_type = 'product';" );
			foreach( $products as $product_id ) {
				$product = wc_get_product( $product_id );
				// Don't flash sale products on sale already
				if( $product->is_on_sale() ) {
					continue;
				}
				$regular_price = $product->get_regular_price();
				$sale_price = $regular_price - ( $sale * $regular_price );
				update_post_meta( $product_id, '_price', $sale_price );
				update_post_meta( $product_id, '_sale_price', $sale_price );
			}
		}
	}
	WP_CLI::add_command( 'cp', 'CP_CLI' );
}