Diamond prices can vary considerably and much depends on the reason why you are buying a diamond as to what type of diamond you want and the price you want to pay.
Unlike gold or other precious metal, diamond is not just sold by weight but also by the quality of the diamond, such as the cut to reflect the best possible sparkle. The color and the clarity also play a part in determining the price of a diamond.
Your budget will determine the price range of the diamond you are looking for. It you are on a restricted budget you might even look at a gemstone such as a sapphire or ruby for example.
But if you only want a diamond then the very least you would pay for a tiny diamond of reasonable quality would be 300 to 500 dollars. The diamond prices then more from there upward almost as far as one can imagine.
Some guidelines may be of help here.
Buy at the top of your budget to ensure you get the best diamond you can in your price range. Going for the cheapest diamond possible is a false economy. You might as well buy a pretend diamond such as a Cubic Zirconia.
If you are buying an engagement ring a good guide is to set a budget of 1-2 months salary. This will usually place you in the market to get a decent 1 carat diamond ring.
You will need to choose carefully. Do not just accept jewelers say so on the quality of a diamond. As him for an independent appraisal by a gemological laboratory. If he refuses go to another jeweler. Many (not all of course) jewelers will sell a stone one or two grades less than promoted. This can mean a difference of several hundred dollars profit. The only way you know what you are getting is if you have the diamond properly appraised.
A good guide as to the diamond prices for various quality diamonds is at Diamond Prices. Here you can find a breakdown of the type of prices you might pay for each quality and weight of a diamond. A good reference for when you go shopping.
Provided one is sensible about diamond prices one can get a really good deal and a great diamond without paying through the nose.