This is an interesting French expression because it adds cher to coûter which translates to “To be expensive” or “To cost dearly”. It’s a little confusing because “cher” is an adverb, whereas “expensive” is an adjective, so the two don’t line up exactly, so “to cost dearly” may be slightly more accurate. This means that cher is invariable because it is modifying the verb coûter, so it does not agree with the subject; in plain(ish) English, coûter is conjugated as normal and cher remains the same.
Ça coûte cher! – That’s expensive! (lit: it costs dearly)
Cette chemise coûte cher ! – This shirt is expensive!
Les maisons dans ce quartier coûtent cher – The houses in this area are expensive.
Il a fait une erreur qui lui a coûté cher – He made a mistake that has cost him dearly.
Ces vacances vont coûter cher ! – This holiday/vacation is going to be expensive!