Learn Portuguese (Portugal): Aqueles vestidos são muito caros. - Those dresses over there are very expensive.

Both words mean "those", but they show different distances. You use esses for things near the person you are talking to. You use aqueles for things that are far away from both of you, like saying "those over there". It is plural and masculine to match vestidos.

Since a dress is typically women's clothing, why is vestidos treated as a masculine word?

In Portuguese, words have a grammatical gender that doesn't always match who uses the object in real life. The word vestido ends in -o, which is the typical ending for masculine nouns. Because it is masculine, the describing words aqueles and caros must also be masculine.

Why doesn't muito change to muitos to match the plural dresses?

In this sentence, muito means "very" and it describes the adjective caros (expensive). When used as "very", muito never changes its form. It only changes to match plural or feminine words when it means "many" or "a lot of" and describes a noun directly.

Why do we use são (from the verb ser) instead of estão (from estar) for the price?