Such sentences are not thought to teach you an idiomatic expression but a particular verb, in this case “kosztować”. So, while you are correct to state that it means the same, the creators and moderators want you to stick closer to the literal translation.
At least that's how I understand it, given that there are numerous similar discussions in earlier lessons.
Well, it's true that we want people to stick closer to the literal translation (as long as it's correct), but we also accept the more idiomatic one, at least unless it has a different, more literal translation. "How much is it" is accepted here. The comment you answered to is very old, apparently from the times the course was still in its beta phase.
I know, and I think that the original poster may not even be active on Duolingo anymore, but this is just a rumour. But in case I was right, which I finally wasn't, I wanted to clarify for future readers who too may have wondered why they were rejected although their answer was correct.
Anyway, thanks for your correction, I will then use the more idiomatic answer the next time this sentence comes up.
By the way, there is a phrase "Po ile jest/są...", which I think is mostly used with food and other things that you need to buy regularly. I wouldn't accept "Po ile to jest?" though, without specifying what 'to' is, this somehow sounds strange to me.
"Po ile jest [currency name]?" is a common way of asking what is the current price of the given currency.
Thanks a lot for this additional information, this sounds useful indeed! But why would »Po ile to jest?« in this raw form sound strange to you, without the clarification of the object you enquired on? To me as a learner, this sounds OK, it does not lack anything. Or is it just strange to the native ear, without any grammatical background? This would be a sufficient reason to me, we have got a lot of such situations in German as well, although more related to past and conditional tenses.