Hi and welcome.
As a present from a grandfather this is priceless, but in re-sale second hand monetary terms, as a single pearl, not much, even though it looks a good shape with clean surface and good lustre.
What it sounds as if happened is that your grandfather came upon a 'wish pearl' set up at a tourist site, where shells are seeded with pearls and the customer picks the shell and opens it and there's the pearl. Great souvenir but only a few pence in re-sale value.
You could get the pearl half drilled to make a more professional looking pendant, since this would free it from its cage.