I am horrible at explaining grammar even in English and never know the names of the parts of speech. I learned VN through immersion vs classes or books so I don't know how to explain the formulas for sentence structure as I studied them after the fact. I don't know if I'll be much help.. But I'll try.
The question were trying to find an answer to is if it's expensive not where the food is.
"the food here" I look at as a whole subject.
I look at it as a simpler formula:
Subject có expensive không?
The subject seems to be "the food here" as a whole, if that helps make more sense?..
I look at it the same as if instead of "the food here" you said "this food" as in "Is THIS food expensive?" you wouldn't say "Thức ăn có đắt không này", right? That's unnatural. You'd say "Thức ăn này có đắt không?" just as you would say "Thức ăn ở đây có đắt không?"
I also can't say "Food có expensive here không?" Because the question isn't "here?", it's "expensive?" To say "ở đây không" you're asking a yes/no question about if it's here.
có.. không (yes or no question) I believe has to end the sentence.
So I also can't say "Food có expensive không here?" because that means "The food is expensive, not here." Không + ở đây = not here
I know this might not make much sense but I figured I'd share my two cents anyways
Subject + có + expensive + không?