Learn Italian: Il biglietto costa duecento euro. - The ticket costs two hundred euros.

In Italian, the word euro is invariable, meaning it does not change its ending in the plural. One euro is un euro and two hundred euros is duecento euro.

How do you build numbers in the hundreds like duecento?

You simply attach the smaller number to the word cento (hundred) to make a single word. Two (due) plus hundred (cento) becomes duecento. This pattern works for all hundreds, like trecento for 300 or quattrocento for 400.

Since we are talking about multiple hundreds, why doesn't cento change to a plural ending like duecenti?

The word cento never changes its ending to agree with plural nouns. It remains exactly the same whether you are talking about one hundred or nine hundred, so it is always duecento, never duecenti.