add_action( 'woocommerce_product_options_pricing', 'genius_set_percentage_discount' );
 
function genius_set_percentage_discount() {
   global $product_object;
   woocommerce_wp_select(
      array(
         'id' => '_pc_discount',
         'value' => get_post_meta( $product_object->get_id(), '_pc_discount', true ),
         'label' => 'Discount %',
         'options' => array(
            '0' => '0',
            '10' => '10',
            '25' => '25',
            '50' => '50',
         ),
      )
   );
}
 
add_action( 'save_post_product', 'genius_save_percentage_discount' );
   
function genius_save_percentage_discount( $product_id ) {
    global $typenow;
    if ( 'product' === $typenow ) {
        if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return;
      if ( isset( $_POST['_pc_discount'] ) ) {
            update_post_meta( $product_id, '_pc_discount', $_POST['_pc_discount'] );
        }
    }
}
  
add_filter( 'woocommerce_get_price_html', 'genius_alter_price_display', 9999, 2 );
  
function genius_alter_price_display( $price_html, $product ) {
    if ( is_admin() ) return $price_html;
    if ( '' === $product->get_price() ) return $price_html;
    if ( get_post_meta( $product->get_id(), '_pc_discount', true ) && get_post_meta( $product->get_id(), '_pc_discount', true ) > 0 ) {
        $orig_price = wc_get_price_to_display( $product );
        $price_html = wc_format_sale_price( $orig_price, $orig_price * ( 100 - get_post_meta( $product->get_id(), '_pc_discount', true ) ) / 100 );
    }
    return $price_html;
}
  
add_action( 'woocommerce_before_calculate_totals', 'genius_alter_price_cart', 9999 );
  
function genius_alter_price_cart( $cart ) {
    if ( is_admin() && ! defined( 'DOING_AJAX' ) ) return;
    if ( did_action( 'woocommerce_before_calculate_totals' ) >= 2 ) return;
    foreach ( $cart->get_cart() as $cart_item_key => $cart_item ) {
        $product = $cart_item['data'];
      if ( get_post_meta( $product->get_id(), '_pc_discount', true ) && get_post_meta( $product->get_id(), '_pc_discount', true ) > 0 ) {
           $price = $product->get_price();
           $cart_item['data']->set_price( $price * ( 100 - get_post_meta( $product->get_id(), '_pc_discount', true ) ) / 100 );
      }
    }
}