Learn Farsi: بعضی از لباس‌ها ارزان است.

Both are actually correct, but they have a slight difference in meaning. Using az gives us ba'zi az lebas-ha, which translates to 'some of the clothes' (referring to a specific group). If you use the Ezafe instead, ba'zi-e lebas-ha means 'some clothes' in general. In everyday Farsi, they are often used interchangeably.

Since lebas-ha is plural, why does the sentence end with the singular verb ast (is)?

In Farsi, when the subject is an inanimate object (not a human or animal), the verb is normally singular. Even though lebas-ha (clothes) has the plural suffix -ha, the clothes are inanimate, so we must use the singular verb ast instead of the plural hastand.

Do we always need to use a plural noun after ba'zi az?

Yes! Because ba'zi means 'some', the noun that follows it needs to be plural to make logical sense. That is why we use lebas-ha rather than just the singular lebas.

Can the singular word lebas mean 'clothes' on its own?

The singular lebas usually refers to a single garment, like a dress, a shirt, or a suit. When you want to talk about multiple pieces of clothing or clothes as a category, especially after words indicating a plural quantity like ba'zi, you should use the plural form lebas-ha.