I answered costly instead of expensive but both adjectives have the same meaning, don't they?
In the meaning of “having high price”, they’re synonyms. (“Dear” comes from Germanic roots, cognate to German teuer, and “expensive” comes from Latin roots.) “Dear” is also used to express affection, but “expensive” isn’t.
I was really asking the question why can Duolingo not accept both 'dear' and 'expensive' as translations of costasach? I don't see that the origins of these words is relevant to how they are used in this translation.