Borrow money from Lorenzo Giovanni, the loan shark in the Hub, until he won't go any higher and then kill him. If the Vault Dweller has the cash to front, they can have a net gain of 1,800 caps.
No one will interfere with killing Lorenzo, except for his guards, Guido and Leone. However, his guards are difficult to take down at lower levels. Completing this side quest can be extremely lucrative as Lorenzo keeps many valuable items in his office such as 2,034 caps in the locker, frag grenades and guns in the bookshelf on top of the initial profit from the loan.