Learn Croatian: Koliko košta jedno noćenje? - How much does one overnight stay cost?

You're right that koliko usually takes the genitive when asking 'how much/many of something' (like koliko vode for 'how much water'). However, in this sentence, koliko is an adverb modifying the verb košta ('how much does it cost'). The phrase jedno noćenje is the subject of the sentence ('one overnight stay'), so it must be in the nominative case.

How is the word noćenje formed?

It is a verbal noun, which is similar to an '-ing' word (gerund) in English. It comes from the verb noćiti (to spend the night), which itself is derived from the noun noć (night). The suffix -nje is added to make it a noun meaning 'an overnight stay.' Because it ends in -e, it is a neuter noun, which is why the number one matches it as jedno.

I've heard that the verb stajati can also mean 'to cost.' Could I use it here instead of koštati?

Yes, you could say Koliko stoji jedno noćenje? Strict grammar guides often prefer stajati for prices, but in everyday spoken Croatian, koštati is universally understood and perfectly acceptable. At a B2 level, it's good to recognize both, but you will hear koštati much more often in casual situations like booking a room.

Since Croatian doesn't have articles like 'a' or 'an', why include here instead of just saying ?