Regarding "The Ransom of Red Chief," are Sam and Bill usually successful in their schemes?

This is a perceptive question? Although "The Ransom of Red Chief" is presented as a funny tale, the moral of the story is clear: Crime does not pay. O. Henry sincerely believed that. He had spent about three years in state prison for embezzlement and never got over the guilt and shame. He wrote under an assumed name and dreaded having his past catch up with him. 
Sam and Bill appear to be middle-aged men who have been trying to make a lot of money through crooked schemes, such as the one Sam mentions at the beginning of "The Ransom of Red Chief." 

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. 

O. Henry probably has Sam refer to the "fraudulent town-lot scheme" in order to show that these two men are career criminals. After all these years, Sam and Bill only have a joint capital of about six hundred dollars--and ultimately they are going to have to pay Ebenezer Dorset $250 of that capital to take their "victim" off their hands. Just the fact that they have so little money at their ages should prove that they are not usually successful in most of their schemes.
Their latest scheme to kidnap a little boy for ransom may be intended as an illustration of the fact that these two clowns are incompetent. They start off with high hopes and end up victimized by their intended victims. They can't even handle a little boy. Their latest caper is just one disaster in a series that leaves them, in middle age, with a net balance of $350. Their "fraudulent town-lot scheme in Western Illinois" will never come off because they needed $1400 before the kidnapping and now they need $1650. Even if Ebenezer Dorset had paid them the $1500 they demanded, they still would have had to add most of their $600 to finance that town-lot scheme.