Learn Spanish (Spain): Son trescientos euros.

In Spanish, when you state a price, the verb matches the thing you are counting. Because euros is plural, you must use the plural verb son (they are). You would only use es for a single unit, like es un euro (it is one euro).

Why does the word for three hundred (trescientos) end in an 's'?

In English we say 'three hundred' without an 's', but in Spanish, the hundreds from 200 to 999 are plural words. You are literally saying 'three hundreds', so tres (three) and cientos (hundreds) combine to make trescientos.

Does trescientos change if I am counting a feminine word instead of euros?

Yes! Unlike numbers 1 through 99, the hundreds in Spanish change to match the gender of the noun. Since euros is a masculine word, we use trescientos. If you were counting a feminine word like libras (pounds), you would say trescientas libras.

Why is there no word for 'it' or 'they' at the beginning of the sentence?

In Spanish, subject pronouns like 'it' or 'they' are usually dropped because the verb itself gives enough information. The verb son already tells us that the subject is plural, so you do not need to add an extra word at the beginning.