"Isn't it expensive?"
That's literally what it says though. 高くないですか = is it not expensive?
Perhaps it's tricky when you're used to thinking of ね only as "isn't it". But ね and か are different final particles, and can't be used together. 高いですね = it is expensive, right? 高いですか = is it expensive?
I translated it as "is it not expensive" (since that seemed to be the literal translation to me) and was marked wrong.
And, although similar, "is it not expensive?" =/= "Isn't it expensive?". I suppose the difference between those two sentences is quite subtle and context dependant - something I'm discovering to be quite prevalent in Japanese. This may just have to be another quirk to keep in mind.
Yeah, I suppose there's no other option than to 'unthink' that connection. :S Within a certain context "isn't it expensive?" can of course reflect the speaker's feeling that it actually is expensive, but that doesn't change the literal translations.
高い "it's expensive" → 高いですか "is it expensive?"
高くない "it isn't expensive" → 高くないですか "isn't it expensive?"