Learn Italian: Le banane costano poco questa settimana.

In Italian, when you talk about a noun in a general sense or as a whole category, you must use the definite article. Since banane is feminine plural, we use le.

Since banane is feminine and plural, why is the word for little poco instead of poche?

In this sentence, poco acts as an adverb describing the verb costano (they cost little). Adverbs never change their endings to match nouns. If it were an adjective describing the bananas directly, like saying few bananas, it would change to match the noun and become poche banane.

Is saying costano poco the normal way to say something is cheap?

Yes! While you can use the adjective economico to mean cheap, Italians very often use the phrase costare poco (to cost little) in everyday conversation to express that something is inexpensive.

Why does the word for this end in an -a in questa settimana?

The word questo means this, and it is a demonstrative adjective that must match the gender and number of the noun it attaches to. Because settimana (week) is a feminine singular noun, questo changes its ending to become questa.