Buying retail will have huge markups. They have significant overhead when compared to online companies. Markups can be as high as 500% in retail stores, in most cases its atleast a 100%. In retail don't get sucked into the line "if you buy today we will cut you a deal." It's not.
If buying online I strongly encourage you to go with a super ideal cut diamond, such as, blue nile signature cut, whiteflash a cut above, brian gavin signature cut, james allen true hearts, et. Comparable retail brands such as Solsafera and Hearts on Fire(as well as Leo, Peerless etc) are significantly more expensive for absolutely no difference in quality(ie: you are buying a brand name).
The reason I say go with a super ideal cut diamond is because its hard to tell a diamonds quality by its numbers so purchasing online sight unseen is hard to do if you aren't going with a super ideal cut. If you buy a super ideal cut you will end up with a fantastic diamond no matter what.
I would highly suggest looking at
www.whiteflash.com and
www.briangavindiamonds.com in addition to Blue Nile and James Allen. Whiteflash and Brian Gavin are HIGHLY reputable and have very very few negative reviews. Another good place to look at is
www.goodoldgold.com but their website is not as easy to navigate as the rest.
Here is a general diamond information site that may help in making a decision.
www.pricescope.com
One final thing I will mention is, diamond prices have been going up a lot this year. So far this year prices have gone up 20-25%. 10% of that spike came during two weeks in March. Prices are expected to continue to rise as demand for diamonds in India and China keeps going up(around 50% of DeBeers siteholders, ie: the people DeBeers sell rough diamonds to, will be based out of India starting in 2013). Still alot cheaper than prices in the 1980s, even without taking into account for inflation.