If I don't know Bob, then what will I gain by giving him a gift? I don't know him anyway. I don't see myself giving my account manager whom I wouldn't recognize on the street any extra money so I don't think Jill should either.

If I am friends with Bob or acquaintances, that obviously changes things. Still, you technically could argue the entire amount over marker average is "due" to Bob's skill and is owed to him. Any amount less would not be fair, you could argue. I would maybe buy him a dinner or something.

If I'm trying to persuade Bob to keep going then that is a different situation entirely.

Maybe I'm wrong, but I would not feel obligated to split the difference with Bob. Would Bob have split the loss if the investment didn't turn out?