Billetten koster femti.

Because billetten means the ticket, while en billett means a ticket.

In Norwegian, the definite article is usually attached to the end of the noun:

• en billett = a ticket
• billetten = the ticket

So this sentence is specifically talking about the ticket, not just any ticket.

The ending -en is the definite article for many masculine nouns in Bokmål. It works like the in English.

So:

• billett = ticket
• billetten = the ticket

This is very common in Norwegian. Instead of putting a separate word before the noun, Norwegian often adds the definiteness to the noun itself.

It is singular.

• billetten = the ticket one ticket
• billettene = the tickets more than one

So the sentence is about one specific ticket.

Koster is the present tense of the verb å koste, which means to cost.

So:

• å koste = to cost
• koster = costs / is costing

In Norwegian, verbs do not change according to the subject the way they do in English. So you say:

• jeg koster would be wrong because the meaning would be strange, but grammatically the verb form stays the same
• billetten koster
• huset koster
• de koster

The present-tense form is still koster.

Norwegian often leaves out the currency when it is obvious from context.

So Billetten koster femti usually means:

• The ticket costs fifty kroner

If you want to be more explicit, you can say:

• Billetten koster femti kroner

Both are natural, but the shorter version is very common when everyone already understands which currency is meant.

Yes. Femti is the number 50.

In this sentence, it means the ticket costs 50, usually understood as 50 kroner unless another currency has already been mentioned.

Yes. This is the normal word order for a simple statement:

• Billetten = subject
• koster = verb
• femti = price / complement

So the pattern is basically:

• Subject + verb + amount

That is the usual and most natural order here.

Not in this context. If you are talking about price, the natural verb is koster.

• Billetten koster femti = The ticket costs fifty

Using er would sound wrong for stating a price. Norwegian uses å koste for this, just like English uses to cost.

A simple learner-friendly approximation is:

• Billetten ≈ bill-ET-ten
• koster ≈ KOSS-ter
• femti ≈ FEM-ti

A few useful notes:

• The tt in billetten is pronounced clearly.
• The o in koster is short.
• The e in femti is like the e in bed.

Pronunciation varies somewhat by dialect, but this will be understood.

It is not necessarily better; it is just more explicit.

• Billetten koster femti = natural when the currency is obvious
• Billetten koster femti kroner = natural when you want to state the currency clearly

So the shorter sentence is perfectly good Norwegian.

Yes. It is mostly used for prices, but it can also be used more broadly in the sense of to cost something.

For example:

• Det koster mye tid. = It costs a lot of time.
• Det kan koste dyrt. = It can cost dearly.

But in a simple shopping sentence like this one, it most naturally refers to money.