Ces billets coûtent cher aujourd'hui.

Because here cher behaves like an adverb, not an adjective.

• With coûter, French usually says: coûter cher = “to cost a lot / to be expensive”.
• When cher is used this way, it is invariable: it does not agree in gender or number.
  • Ce billet coûte cher.
  • Ces billets coûtent cher.
  • Cette place coûte cher.

If you use cher as an adjective in front of the noun, then it agrees:

• un billet cher
• des billets chers
• une place chère

So in your sentence, cher stays in the basic form because it’s part of the fixed expression coûter cher.

Yes, you can say Ces billets sont chers aujourd'hui, and it’s correct.

Nuance:

• Ces billets coûtent cher aujourd'hui.

  • Focuses on the price and the act of costing.
  • Literally “These tickets cost a lot today.”

• Ces billets sont chers aujourd'hui.

  • Focuses more on the quality “(being) expensive”.
  • Literally “These tickets are expensive today.”

In everyday speech, both are very natural and the difference is subtle. Coûter cher is just a very common, idiomatic way to comment on price.

Billet can mean both:

• un billet = a ticket (for a train, concert, plane, etc.)
• un billet = a banknote / paper money (e.g. un billet de 20 euros)

In Ces billets coûtent cher aujourd'hui, the meaning depends on context:

• At a station or online booking site: probably tickets.
• In a conversation about money or currency: possibly banknotes (e.g. rare old banknotes that are now valuable).

Without context, a learner will usually be told it means tickets.

The verb agrees with the subject:

• Subject: Ces billets → plural (they).
• Verb: coûter → 3rd person plural present: ils coûtent.

So you get:

• Ce billet coûte cher. → singular
• Ces billets coûtent cher. → plural

Note: the final -ent in coûtent is silent, so coûte and coûtent are pronounced the same /kut/. Only the written form changes.

Ces is a demonstrative adjective meaning roughly these / those.

• ces billets = these tickets (the ones we’re talking about or pointing at)

By contrast:

• les billets = the tickets (more neutral, just “the tickets” in general or already known in context)

Use ces when you want to indicate specific ones, often close in time, space, or context:

• Ces billets, là, sur la table, coûtent cher.
• Les billets coûtent cher (can sound more general, “tickets are expensive”).

For demonstratives, French has:

• Masculine singular: ce (or cet before a vowel sound)
• Feminine singular: cette
• Plural (masc. or fem.): ces

So ces is the only plural form, used for any plural noun:

• ces billets (masculine plural)
• ces places (feminine plural)
• ces hommes, ces femmes, etc.

It doesn’t change in form for gender in the plural.

Yes. Both are correct:

• Aujourd'hui, ces billets coûtent cher.
• Ces billets coûtent cher aujourd'hui.

Differences:

• Aujourd'hui, ... puts more emphasis on “today” as the time frame.
• Ending with aujourd'hui is very natural and neutral in spoken French.

Both word orders are common and acceptable.

Yes, slight difference:

• aujourd'hui = today (focus on the day as a whole)

  • Ces billets coûtent cher aujourd'hui.
    → They’re expensive today (maybe compared with other days).

• maintenant = now (focus on the present moment)

  • Ces billets coûtent cher maintenant.
    → They’re expensive now (perhaps they were cheaper earlier).

Both can sometimes overlap in meaning, but aujourd'hui is more natural when you’re talking about today’s prices.

Traditionally:

• coûter is written with û to mark the vowel sound /u/ and to reflect older spelling (it used to be spelled with an s: couster).

According to the 1990 spelling reforms, couter without the circumflex is allowed, but:

• In practice, most people still write coûter, coûte, coûtent, etc.
• For learners, it’s safer and more standard to keep the û.

So write coûtent rather than coute or coutent in formal or careful French.

Yes, cher has several meanings depending on position and context:

1. Expensive

   • Ces billets coûtent cher. → They’re expensive.

2. Dear / beloved before a noun

   • Mon cher ami → My dear friend.
   • Chère Madame, ... → Dear Madam, …

3. Dear as a form of address

   • Cher Pierre, (at the beginning of a letter or email)

In coûter cher, it only means “expensive”, not “dear (personally)”.

No, that sounds wrong in French. With coûter, you don’t normally use beaucoup directly like that.

Correct options:

• Ces billets coûtent cher aujourd'hui.
• Ces billets coûtent très cher aujourd'hui. (very expensive)
• Ces billets sont très chers aujourd'hui.

So, to intensify, you usually add très (or vraiment, horriblement, etc.) before cher, not beaucoup after coûter.

Key points:

• ces
  • billets: there is a liaison → ces is pronounced /sez/
    • /sez biljɛ/
• billets: the final -ts is silent → /biljɛ/
• coûtent: pronounced /kut/ (same as coûte in speech; the final -ent is silent).
• cher: /ʃɛʁ/ (in France) or /ʃɛʀ/ (r slightly different in some regions).
• aujourd'hui: /oʒuʁdɥi/

Whole sentence (French from France):
/sez biljɛ kut ʃɛʁ oʒuʁdɥi/

There is no liaison between billets and coûtent, because coûtent starts with a consonant.