Sam and Bill are con artists. They are thieves and kidnappers as well, as this story will show. They have six hundred dollars with them at the beginning of the story. They figure that they need two thousand more in order to pull off another scam in which they will defraud people of more money:
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.
Sam never tells us (the reader) what the town-lot scheme is. However, given that these guys are con artists, thieves, and essentially professional criminals, it would probably have something to do with selling lots that do not belong to them. It may be a case where a new town is in development and a financier or the government (state or federal) is selling the plots in what will be the new town. Knowing where and when this would happen (Western Illinois, for example), Sam and Bill could go there before the real administrators get there. They could pose as the sellers, sell plots of land to people intending to move there, and then leave with the money before the real sellers get there. This is an example of a "town-lot scheme" and is a possible scenario of what Sam is talking about.