Look at these 2 contrasting examples:
La jupe coûte 30€. - Cette jupe est chère.The skirt costs €30. - This skirt is expensive.
La jupe coûte 30€. - C'est cher !The skirt costs €30. - That's expensive!
- In the first case, the adjective (describing word) chère relates to a specific noun (la jupe) present in the sentence, therefore the regular rule of agreement applies.
- In the second case, we're still talking about the skirt, but commenting on its price in a general context, using c'est (that is), hence the use of the masculine form of the adjective (cher).
When a standalone adjective is used after c'est, it will always be in the masculine form.
Here are more examples:
C'est ennuyeux.It is boring.
C'est génial !It's great!
C'est gratuit.It is free.
Also see Describing things in French with c'est = it is
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