There are the ‘1+1 free’ champagnes again. But are they really worth their little money?

There is December, there are the champagne prices. With traditional ‘1+1 free’ promotions. Where do those dirt-cheap bottles suddenly come from? Do supermarkets actually make anything from this? And above all: are those champagnes tasty? We asked a few experts, including our resident sommelier Sepideh Sedaghatnia.