I've always been skeptical about online gem pricing, at least it's never seemed like much of a discount to me.
For really good stones, VVS and better, F and better, cut good or better, a retail price of $8,000/carat is likely so if you can get it closer to $5K you're doing well. And like the others have said there's big variability depending on the stone, the source, etc. Don't be shy about going for a "used" rock, I've heard that a surprising number of used diamonds get recycled back through the "new" channels anyway.
One handy trick, bring a sheet of high quality, premium white bond paper with you. Crease it down it's length and set the stones in the crease. Even the slightest variation in color will be immediately apparent to your eye. Otherwise the merchant will be displaying the stones on a gray, black or off white mat that can disguise the color of the stone. Personally, I've always found that color was more important than an occasional inclusion, for instance, what stone does she already have and what will this new one be sitting next to? If she's wearing a D color diamond and you buy her a G or an H it will be noticeable.
Good luck and remember: when in doubt get a cubic zirconia and spend the money you've saved on hookers!