When people ask what their pearls are worth I often suggest researching "sold" prices of similar objects-- because ultimately the only monetary value an object has is whatever people are willing to pay for it. (An object no one wants to buy has no monetary value, though it may have a great deal of emotional value to its owner.)
Since natural pearls are not abundant, it might be hard to research recently sold examples of pearls like yours--but you can look at what similar natural pearls are listed at on the websites I mentioned. Then remember that to make a profit, the vendor would have to buy them from you at a lower price than they plan to sell them for.
Sorry that I can't provide a better answer. Maybe someone else has more experience in this.