There are no set standards of pricing diamonds, as we have seen with the four parameters of diamond evaluation. Some colors are rare, while others are relatively available, some are of good color but poor clarity and vise-versa, and large carat-weights are so uncommon their prices are hugely disproportional to smaller carat weights. There is no clear list of prices to be gained from this, because each variable affects pricing and to confuse matters further; mounting adds a whole new dimension to the cost. The diamond market fluctuates with new cutting techniques that change the quality of the stone and some types aren't even been mined anymore and their price have skyrocketed over the years.
If you are buying a diamond one carat weight or more, have it appraised or buy it only after proper documentation has been presented to you. It is also in your best interest to ask that it be unmounted, re-weighed, and then we remounted. Grading a mounted stone is utterly impossible, so don't believe anyone who suggests it. If the piece contains many small stones, then they must have been graded and weighed separately before mounting and this too, should be documented and signed for.
However, there are general guidelines for pricing that can give you broader perspective on the subject and educate you to the top and bottom markers for prices in various stones. Since the most common shape to be found is the round, brilliant cut, this will be your first parameter. Any other shape of diamond will sell for about 10% less.