look for a reputable jeweler and ask. He will give you ranges for quality and carat wt. then you can check on line and see what range your quality and carat wt. is currently selling for and judge whether or not your jeweler is acceptable to you. Value is subjective, mind you. Exercising your research should help you negotiate for your stone. After several trials you may know enough to know whether you are given a fair offer or not.
So I typed gem value chart in, I do not know anything about the chart but I included the chart here. Prices listed are per carat I assume.