Why does Duo dislike the word "costly"? It is perfectly good English word meaning expensive! And it has fewer letters to type!
In your case, you need to tell Duo your answer was correct. There are multiple ways to say the same thing. Your answer is less common, but still correct. So they didn't think of it yet and you need to tell them :)
It takes them a few weeks to get to everyone. Don't be too frustrated... they really do get there... eventually ;)