"Isn't it cheap?"
This duolingo question throws me off because when someone says "isn't it cheap?" they really mean "やすいですね" and not "やすくないですか?"
I believe "やすいですね" seeks agreement as in "isn't it cheap? Don't you agree?" whereas "やすくないですか" seeks clarification as in "isn't it cheap? I thought it was cheap..."
When you say yasuku nai desu ka you're really emphasising that the thing you're taking about isn't cheap. It's like you're commiserating/seeking empathy from others in a way. Think of it like when you take your whole family to the movies and after the ticket prices, and all the treats per person, drinks, an icecream each, popcorn, Tommy likes peanut MnMs but Jill doesn't, after all the tickets and communal and individual goodies are tallied up the final total floors you and you might turn to people behind you in the line or just say aloud to no one in particular - Phoarrrrr!!!! It's NOT cheap, IS IT??!?!!! To which you would no doubt get murmurs of heartfelt agreement and sympathy from other parents.
Whereas if you were to say yasuku nai desu ne, it's a very gentle, everyday way of seeking confirmation. You could translate it as it's not cheap, is it? (tone is important here). Or it's not cheap, eh? That's the difference : )
Hey asdf, I think you've misunderstood what I was trying to say - I was attempting to explain the feeling behind the Japanese and the difference between ~nai desu ka and desu ne in English. I wasn't trying to explain English. Rather I was trying to explain what the Japanese meant in English by appealing to English phrasing that is similar to the Japanese meaning.
Given your explanation, the connotation of the English phrase "isn't it cheap?" (and all "isn't it ?" phrases, at least in AmE) would mean that this isn't the best answer for this Japaniese phrase even if it literaly is correct. From my English speaking mind, "isn't it " is an empathetic form of "is it _" where one asks for agreement or approval ("isn't it great" where the speaker thinks that "it" is great and is asking if the listener agrees; kind of similar to "is it great or what") or one is clarifying what one thought ("isn't it supposed to be on" where the speaker thought that "it" was ment to be on but has come across some knowledge that brings doubt about that assumption and is asking for clarification. Although this does match up to what you describe, it is does not feel nearly as forceful and is almost neutral to me without a very strong tone behind it). There also seems to be a trend for positive things to be of the approval nature while negative things to be of the clarification nature ("isn't it expensive" feels like the speaker may not want to do something because of a preceived cost while "isn't it cheap" feels like the speaker is saying that the object is quite inexpesive for what it is. Although, cheap can also be negative in which case it feels more like the speaker may not want to do something because of a preceived low quality).
Doesn't this actually reflect that you can say it both ways, in english as in japanese? This is the -ka version, and the -ne version is 'This is cheap, isn't it?'
I had a really hard time with this type of sentence construction when i first learned it. As another person mentioned, it was confusing to me because my instinct would be to express this idea by saying やすいですね rather than やすくないですか. It helped me to think of the strict literal translation: 1) it's cheap, isn't it? 2) isn't it cheap? Both versions convey the same general idea, but they're slightly different ways of saying it.
I think the other poster's comment about the difference in tone makes sense, too, and it's kind of difficult to get used to because it doesn't readily carry over like that in English.
Quick story: first time i encountered it in class, we were talking about the day. Sensei asked 月曜日じゃないですか, is it not (or, isn't it) Monday? and I was just like... um... yes, it is Monday. Why are you saying it like that? It sounded as if she was confused, expecting it to be a different day, and i thought, why not say "it's Monday, isn't it?" Or "today is Monday, right?" She explained it as being a kind of gentler/more polite way to confirm the date rather than using 月曜日ですね.