The quality of the Emerald dictates its price as with other precious stones. With Emeralds however, the price accelerates greatly with the larger stones. For example, a 6 carat Emerald will be much more per carat than the same quality 2 carat Emerald. It can be as much as 4, 5, or 6 times as much as the large Emeralds are just so hard to come by in good quality.
Value depends a lot on color and clarity too. The most sought after color is a slight bluish green in a medium dark tone. Clarity is important, but inclusions are acceptable with Emeralds, unlike diamonds and other gems.