Learn Polish: Płacisz sześćset złotych za ten mały pokój? - Are you paying six hundred zlotys for this small room?

It is a combination of sześć (six) and set, which is the genitive plural of sto (hundred). In Polish, hundreds from 500 to 900 are formed by adding set to the base number, so 600 literally means six of hundreds.

Why is the currency written as złotych instead of złote?

In Polish, numbers from 5 upwards require the noun that follows them to be in the genitive plural case, which is used for quantities. Since sześćset is larger than 4, the currency złoty changes to its genitive plural form, złotych.

What case is used after the phrase płacisz za?

The verb płacić (to pay) pairs with the preposition za (for) and requires the accusative case for the item you are buying. This is a set phrase worth memorizing: płacić za plus the accusative case.

If we need the accusative case for the room, why doesn't ten mały pokój change its endings?

Because pokój (room) is a masculine inanimate noun, meaning it is a non-living object. For masculine inanimate nouns, the accusative case looks exactly like the basic dictionary form (nominative). Therefore, ten mały pokój doesn't change its endings.