Stiftung Warentest leaves the price out of its quality grade. Teaching material developed with the foundation gives the reason. Da der Preis nichts über die Produktqualität aussagt, because the price says nothing about the quality of the product, it is not part of the rating. The price is still shown, as important information for the purchase.
As the foundation put it in 2003: Teurer ist nicht gleich besser. More expensive is not the same as better.