Translation:Isn't it expensive?
高い, by itself, already means "it's expensive". Likewise, 高くない means "it is not expensive", if it's a statement. But while English flips the verb and subject in order to make it a question ("is it not expensive?"), Japanese only changes intonation, since it doesn't require an extra verb. The ですか is added to make clear it's a question and not a double negative.
I think the original comment was trying to ask if it would be more properly translated as "Is it Not expensive?" (In english if i answered yes to this, I would be saying that it is in fact "Not expensive")
Verses Duo's translation of "Isnt it expensive?" (Which, if i answered yes; i would be saying that it is in fact expensive)
"Isnt it expensive?" Seems more like it would better fit the translation of 高いですね than 高くないですか
I think that in English it can also depend on intonation and context. For example:
Person A: "oh, that's cheap!". Person B (with less money): "isn't it expensive?" (as in, "do you not think it's expensive?"). In this case 高くないですか is a genuine question, asking about whether the object is in fact "not expensive" according to the listener.
Person A: "isn't it expensive?". Person B: "yeah, that's way too much". In this case, it's actually meant as more of a statement ("I think it is expensive"), and said under the assumption that the listener agrees with the speaker. In Japanese that is usually expressed as 高いですね.
So does Takakunai desuka mean that something is expensive or inexpensive? I think my understanding is the same as yours: 高くないですか is a genuine question, asking about whether the object is in fact "not expensive" according to the listener. While to say that something is expensive, and looking for agreement, you would use 高いですね.
That's a matter of education and upbringing. Most English speaking people use the words they're familiar with in whatever context regardless of how inappropriate or misleading it comes across.
Some of us choose words to communicate more than their definitions imply... calling something "cheap" instead of "inexpensive" is very deliberate. Like using "plump" or "fat" versus "overweight."
I disagree with the translation "Is it not expensive?" since it implies that the person asking the question is persuading the other person that to agree that it is expensive (instead of otherwise). I think the usage of a Question Tag would provide a better translation:
It is not expensive, is it?