Of course the price in grocery stores varies depending on the company, but either bananas or national bananas (in Spanish, "plátano/s") are all usually always below 2€/Kilogramme. For instance, in the one I'm checking they're at 1'55€/Kilogramme and 1'9€/Kilogramme for the national product (check "Plátano de Canarias"). If, instead of supermarkets, you go to traditional markets they may be a few cents more expensive, depending on the product the flavour difference is more noticeable.