Learn Russian: Билет стоит шестьсот рублей.

It depends on where you put the stress! сто́ит (with the stress on the first syllable) means 'costs'. If you stress the last syllable (стои́т), it means 'stands'. In written Russian, they look exactly the same, so you have to rely on context.

Why is the number 600 written as one word (шестьсот)?

In Russian, numbers in the hundreds combine into a single word. Шестьсот is literally 'six hundreds', where сот is an old plural form of сто (100). You will see this exact same pattern in numbers like пятьсот (500) and семьсот (700).

How is шестьсот actually pronounced?

Even though it is spelled with a soft ть followed by an с, native speakers usually blend these consonants together. In everyday conversation, you will almost always hear it pronounced as 'шыссот' (shyssot) rather than carefully sounding out every letter.

Why is the word for roubles (рублей) in the genitive plural form here?

When counting in Russian, the noun's ending depends on the last digit you say. Quantities ending in 5 through 9, as well as 0 (like шестьсот), must be followed by the genitive plural. Because рубль (rouble) is a masculine noun ending in a soft sign, its genitive plural becomes рублей.