Hard Q... because the answer depends on what you are looking for. As a rule of thumb, the lower the total cost of something, the higher the respective markup as a percetage.
There was some discussion about Tiffany's 1 ct or so classic e-rings. You may find older threads and see how the chat evolved (quotes are posted here and there as well). As far as I remember, the consensus decided on 40% markup or so compared to the best of the best elsewhere (ideal cut, H&A diamonds and some comparably well made setting). It is much easier to decide with a quote in hand how much someone's stamp of approval is worth. This is just for a reasonably simple design for a diamond solitaire... what do you have in mind ?
Not that there are that many directly matching refferences there were a couple of relatively similar purchases around here (that I remember of, no index), but none from a brand.
I don''t think you can really compare HW & Graff to something like a Blue Nile. That''s like wondering what the markup of a Bentley is in comparison to a Honda.