I agree with you, except that Duo doesn't know if you've actually understood the question. So, it'll err on the side of caution unless your answer clearly demonstrates that you have understood. This is why it'll sometimes accept idiomatic translations.
Going the other way, because it is teaching Duo will often not accept literal translations for idiomatic questions so it is possible to answer the question correctly and still get it wrong.
The problem is that these questions are not marked as idiomatic beforehand, so you have to learn which phrases aren't literal by trial and error (like "estou com vontade" :p).
For this question, I wouldn't flag or report it- Your answer is understandable but not quite correct- Like you said, it's crap English :)
Thanks! Right, then the "correct" answer is completely wrong! The correct answer should then read "How much does pineapple cost a kilo/How much does pineapple a kilo cost". In English it's important to distinguish between what Duo lists and what I wrote, since price a kilogram is about a ratio.