Gillian explains to the lawyer why he started to laugh upon being handed his inheritance. He explains that the sum he has been given, which is $1,000, is an “unusual amount.” He goes on to explain that
If it had been ten thousand a man might celebrate with a lot of fireworks.
What he is saying here is that $1,000 is not enough money to really make a difference in his life. He does not feel like a wealthy man having been bequeathed this relatively low sum. The reason for his laughter relates to the fact that he and the lawyer have gone to the trouble of meeting and presumably signing documents—all for an amount of money that really doesn’t mean much to Gillian. The laughter relates to Gillian’s surprise and confusion at being left this specific amount.
We get more detail about why Gillian thinks this inheritance is funny when he talks to Old Bryson at his club. In his conversation with Old Bryson, Gillian describes the situation as “sad and funny,” and when Old Bryson points out that Gillian’s uncle had been worth around half a million dollars, the $1,000 bequest starts to feel more like a joke than a real gift. Of course, as it turns out, the initial gift of $1,000 provided a test which Gillian had to pass in order to be bequeathed a far greater sum.