"wagh'a' paqmeyvam? HISlaH. waghqu' bIH."
Translation:Are these books expensive? Yes. They are very expensive.
Since Klingon words are so short it is easy to have a one letter difference convey a huge difference in meaning. That is part of why we wish Duolingo would be stricter about missing qaghwI'mey and other spelling errors that it treats as typos. However, in this sentence the plural marker might be a clue that you had misread it. Remember that for beings capable of language, the plural marker should be -pu' and so the -mey in this case might have given you reason to stop and rethink your first reading of the sentence.
Another insidious result is things such as tlhIngan bIlegh as a translation of "you see the Klingon" being treated as "almost correct" because it thinks it's just one letter off from an alternative correct answer tlhIngan bolegh, even though thinking that bI- is the appropriate suffix for "subject = you (singular), object = him" should result in this being considered an error.
But as you say, we can't tune the strictness of the grading.