Learn Portuguese (Portugal): Os limões são baratos.

Most Portuguese nouns that end in -ão form their plural by changing the ending to -ões. So, o limão (the lemon) becomes os limões (the lemons). While there are some exceptions that change to -ãos or -ães, -ões is the most common pattern you will see.

The word são also ends in -ão. Does it follow the same plural rules as limão?

No, because são is a verb, not a noun! It is the 'they' form of the verb ser (to be). The rules for changing -ão to -ões, -ãos, or -ães only apply to nouns and adjectives. For verbs, an -ão ending usually means the verb is already in a plural form (like 'they are').

Why do we use são (from ser) instead of estão (from estar) to say the lemons are cheap?

In Portuguese, you often use ser to describe a general characteristic or the normal price level of an item. If you say os limões são baratos, you mean lemons are generally a cheap fruit. If you wanted to say they are temporarily cheap today because of a sale, you would use estar and say os limões estão baratos.