By the end of the story, Sam thinks he's getting a bargain when Red Chief's father says he'll take the boy back for only $250. Sam's feelings could be described as terror of the tyrannous child and desperation to get him out of his life.

Sam starts off his story, which he tells in conversational voice, by saying that he and his partner Bill Driscoll thought it a "good thing" to kidnap the son of Ebenezer Dorset, a wealthy and prominent man in the town of Summit. Even when the freckle-faced fiend of a child throws a brick in Bill's eye while they are kidnapping him, Sam persists in thinking their plan will work out. It is only after the child has terrorized them, they have kicked him out, and he has returned that Sam begins to believe the kidnapping was a horrible idea. The comic story is a classic example of irony, in which expectations reverse themselves. The kidnappers become the kidnapped, a child controls adults and the would-be criminals gratefully pay the father to take back his son.  

In O Henry's "The Ransom Of Red Chief", Sam is the "brains" (if there are any brains) of the operation.  At the beginning of the story, he reasons that given the size of a child that a child would have to do what he says.  He also wrongly assumes that "Red Chief" will be afraid of them and that the parents will gladly pay a ransom to get him back.

He is wrong.

At the end of the story, he discovers that the boy is both cunning and conniving- a trait that given his parents, the Dorsetts, stunning counter demand, he has obviously inherited. Nothing he assumed about the boy was correct.