add_filter( 'woocommerce_get_price_html', 'ts_custom_price', 10, 2 );
function ts_custom_price() {
    global $product;
  
  if ($product->is_on_sale())
  {
  if( $product->is_type('simple') || $product->is_type('external') || $product->is_type('grouped') ) {
    $regular_price   = get_post_meta( $product->get_id(), '_regular_price', true ); 
        $sale_price   = get_post_meta( $product->get_id(), '_sale_price', true );
    
      if( !empty($sale_price) ) {
      echo '<h4 class="cana_pred_slevou"><small><strong>Před slevou:</strong></small> '.$regular_price.'</h4><h3 class="cena_po_sleve"><strong>Po slevě:</strong> '.$sale_price.'</h3>';
    }
  }  
} 
}