It's possible, even likely, that all these places can buy the same diamonds. As in, the exact same diamond. Blue Nile will often have the best price, so if you take print-outs to a shop, you can flat-out ask if they can buy that same diamond and price match.
As for the setting, unless you are spending a lot of money and getting something customized, they will all have the same catalogs under the desk. They order the stone and the setting and then put it together for you. They may even show you the catalog and let you pick out a setting you like. They will then call the company for the metal spot price that day and quote you a price - it will probably be triple-keyed, which means three times what they are paying the casting manufacturer. So obviously they'll need a cut, but that should give you an idea of where you are negotiating from.
Ideally, and this is just my personal preference, you would go to a local metalsmith/jeweler and design something unique that was within your budget. For instance, my wife's ring doesn't have a diamond, it has two sapphires and a center pearl. My wedding band is sterling. My point is, I hope you possess the self-confidence to do something different if you want to do something different. It paid off for me, my wife adores her ring and gets complimented on it all the time (including from high-end jewelers we met in New Mexico). Plus, unless your budget is huge and you can get a monster custom diamond ring anyway, this allows you to support local business and trained artists instead of the cookie cutter crap you see at Jared's.