<?php
add_filter( 'woocommerce_get_price_html', 'my_price_html', 100, 2 );
function my_price_html( $price, $product ){
  global $product;
  $return_string = $price;
  if ( $product->stock_status == 'outofstock' ) {
  	  $return_string = $price .' &nbsp;(Brak w magazynie)';
     }
  return $return_string;
}
}