I asked this particular shaila once when it was relevant to me. The example of the shoe store is analogous to my situation, so I’ll use it.
Marshall’s sells the shoes off the shelf; the customer fits himself and pays for it. The typical “heimish” shoe store has a professional fitter who dedicates time and expertise to helping the customer decide what to buy based on his particular needs, and will advise as to quality, comfort, and durability, and make sure the shoe fits properly.. He can therefore charge more.
On the other hand, if there are two competing grocery stores, one cannot knowingly charge more than the other on the same package of cookies. they could charge more if the store is upper scale, with nicer decor, valet parking, etc., because the entire shopping experience is taken into consideration.
As far as your point regarding subjectivity, a dayan would have to decide. Very possibly, an industry which is very service oriented is much less subject to ona’ah than one which is product oriented, and the line differentiating between them can sometimes be blurry.
To illustrate a clear cut case which I think anyone would agree to, I don’t think anyone would doubt that a full service gas station can charge more than a self-serve station.