Actual Diamond Values

Markups vary radically from store to store. Some like Blue Nile's are so low that there is no real room to negotiate. Mom and Pop stores provide better service and need a higher markup, but some are minimal and some expect you to negotiate. Maul chains have very high markups and expect you to negotiate. Some stores charge the bare minimum for a stone but hike up the setting prices and won't let their stones out the door without a setting, some do the exact opposite. So there's really no rules.

A better plan is to start with the kind of store you want to work with (long drive, all over the net, in the neighborhood), find a good one and get an idea of how their pricing levels compare to other merchants. Then you can suggest what you consider to be a fair markup for the level of service they provide.