Because this item asks you to translate an idiom that does not exist (literally) in English, the program ought to accept any answer that means the same: e.g. costs a pretty penny, costs the moon, costs an arm and a leg, costs a fortune, costs a whole lot. There's no way to know that "arm and a leg" is the only right answer here when you are translating it
YES, I translated it directly to "an eye of the face," thinking it was strange, but perhaps just a foreign saying... maybe we should have received a heads-up on this. This was a definite WRONG answer for everyone unless you were somehow already familiar with this very unusual translation.