Why does Bill ask Sam to reduce the ransom amount?

Bill asks Sam to reduce the ransom amount because he believes that it would be cruel to expect a father to pay so much for his ill-behaved child's return. It is also likely (based on his interactions with Johnny) that Bill has little faith Johnny's father will pay the two thousand dollars they are requesting.
To placate Bill, Sam asks for a ransom of fifteen hundred dollars in his letter to Ebenezer Dorset. However, Bill's fears are realized when he and Sam receive correspondence from Johnny's father. In his letter, Ebenezer Dorset tells the men that their ransom demand is much too high. Instead, he makes a counteroffer, requesting that the men pay him 250 dollars cash in order to take Johnny back.
Ebenezer finishes up his letter with a warning: the men had better bring Johnny back at night, so that the neighbors do not see the boy returning. Ebenezer hints that the neighbors will not be too pleased to see Johnny home and might take out their anger on Bill and Sam. This humorous story ends with Bill running as fast as he can towards the Canadian border after paying 250 dollars to Ebenezer Dorset to take Johnny back.