This is a question that I'm sure has been asked for many ages and we still have people asking this very question. For a better understanding of the answer, it takes a bit of research and looking for the perfect diamond for the individual. The most important aspect in answering the above question is the 4 C's of the diamond - Carat (weight), Clarity (how many imperfections exist inside and outside the diamond), Cut (the symmetry and cut quality of the diamond, from a rough stone, to a finished, cut and polished diamond) and lastly, Color (from the clear, whitest to yellow / brown).
To put this into a working perspective, you can purchase a 2 carat diamond, with very high (excellent) specs and it will cost over R1,200,000 or $80,000! Comparatively, a 5 carat diamond, with lower specs, yellowish color, good clarity etc. would set you back R750,000 or $50,000. Surely the larger diamond would cost more? Such a difference in carat size (2 carat and 5 carat). One is significantly larger than the other. It is purely a matter of the combination of the 4 C's and other market factors at play.
Have a look at this chart of estimate diamond prices. This is a rough guide on the price of diamonds. In the image below, you can see that there are three different specification columns with large differences in pricing. A Low, Average and High Spec :
Let's have a careful look at the 5 carat (last row) at the bottom. Can you see how quality in a diamond have a huge impact in variance in price? Remember, there are roughly 7500 different options when it comes to one single diamond - this is very specific and largely a consumer based decision.
When purchasing a diamond - we would advise you follow the 4 C's and pick a stone that is individually suited. You may also find that you find a perfectly cut VVS1 diamond, with a fairly good color and because of the perfect cut, the stone actually shines better than a flawless diamond which is not cut very well. In everything, and all cases, we say CUT is the most important factor when grading diamonds. If the cut is poor, the diamond has poor reflection of light and brilliance.
Always be well informed and ensure that you inspect the diamond carefully before making a purchase.