Fuck the haters. It's not that hard to buy diamonds. Each of the 4 C's will affect the cost. Cut - how well it reflects light (good, very good, ideal, etc.) Clarity - how many imperfections (inclusions) are in the stone (Slight inclusion 1 & 2 (SI1; SI2) very slight 1 & 2 (VS1; VS2) very very slight... up to flawless Color - actually how much color the diamond has (starts with D, E, and F as colorless and goes up the alphabet from there) Carat - Size of the stone
Play around on bluenile.com to get an idea of what a loose stone goes for and it will help give you an idea of your universe. I have a guy in Houston (PM me if you want his name), but bought a stone from Zadok and found they didn't make much on the stone at all (2 to 3%). They made their money on the setting.
When you're buying make the jeweler show you a bunch of diamonds in different cuts, clarity, colors (you may find you are happy with a lower standard on one of the Cs and be able to up another). Make sure you take them outside to look at in the sunlight (if they won't do that tell them to fuck off an go somewhere else).
The setting is another animal all together. Figure out what she prefers (solitaire; Tiffany; halo; cathedral; etc.) and what metal you'll be working with.
That's all I've got off the top of my head, but good luck to you.