In O. Henry's short story, "The Ransom of Red Chief," Sam is the narrator of the story. He is part of a pair of men (Bill and Sam) who have come up with a scheme to get money. Readers are told only the following about their scheme: 

"Bill and me had a joint capital of about six hundred to pull off a fraudulent town-lot scheme in Western Illinois with." 

The two men come up with the idea to kidnap a kid in order to earn a ransom. They are hoping to get two thousand dollars in addition to the six hundred that they have. We know from what Sam says that this is not legal--it's a scam. The words "fraudulent" and "scheme" tell us this. What is not clear from the text is why they need money for this scheme and what exactly a town-lot scheme is. Readers need to make inferences. 

During the time period in which this story is set, when a town was being developed it was often divided into town-lots to be sold. Readers can infer that Bill and Sam are trying to sell town lots that either do not exist or aren't theirs to sell. 

The humor in the story comes from the fact that they kidnap a mortgage financier's son (a bit ironic, isn't it? or is that part of the scheme?) and believe they will easily get the ransom. When they meet Johnny, also known as Red Chief, he fights hard and hits one of them with a brick. They threaten to raise the ransom immediately by five hundred dollars for this act. However, Red Chief gives them so much trouble--incessant talking, hyperactivity, crazy acts--that they lose hope of ever receiving a ransom. At one point, one of the partners begs the other not to leave him alone with the child. 

When they receive a response to their ransom note from the father of Johnny, this is what it says: 

"Gentlemen, I received your letter today in regard to the ransom you ask for the return of my son. I think you are a little high in your demands, and I hereby make you a counter-proposal, which I am inclined to believe you will accept. You bring Johnny home and pay me two hundred and fifty in cash, and I agree to take him off your hands." 

They started with $600. They hoped to make $2,000, which would have brought their total to $2600. They ended up paying $250 to get rid of Red Chief. They were the ones who were scammed. 

In "The Ransom of Red Chief," Bill and Sam need $2,000.00 more than the $600.00 that they possess in order to start their land scam in Western Illinois.  Of course, the irony of the opening words about the small town of Summit, Alabama, and the ease with which they will pull of the kidnapping job cannot be missed.

We knew that Summit couldn't get after us with anything stronger than constables and, maybe, some lackadaisical bloodhounds and a diatribe or two in the Weekly Farmers' Budget.  So, it looked good.

As it turns out, Sam and Bill lose $250.00 to buy back their freedom from "Red Chief."  Running out of Summit, Sam has trouble keeping up with Bill, even "as fat as he was."--A most amusing tale!

Bill and Sam want to start a land fraud scam and to get it going they needed an additional $2,000 to add to the $600 they had.

"Bill and me had a joint capital of about six hundred dollars, and we needed just two thousand dollars more to pull off a fraudulent town-lot scheme in Western Illinois with." (Henry) 

That is the only explanation that I could find to explain what Bill and Sam need the ransom money for, they are professional criminals, that is how they live.