"This banana is cheap."
Translation:هٰذِهِ الْموزة رَخيصة.
When you say موز, it's like talking about bananas in general, not specific. E.g: I love bananas. موز is also grammatically treated as a singular. When you say موزة, you are talking about one banana. E.g: I am eating a banana./I am eating the banana. If you want to talk about a group of specific bananas, you would say موزات. E.g: These bananas are delicious.