can be found in Shakespeare's King Lear, in which Regan and Goneril, Lear's two oldest daughters, try to argue him out of bringing his 100 knights with him to stay with them; what he saw as a necessary condition to holding onto some power, they saw as an unnecessary expense. Just as Fanny reduces John's intention of giving 1000 pounds apiece to the Dashwood girls to the preposterous suggestion that the girls will be more able to give John something, Regan and Goneril start by reducing the number of knights Lear can have, eventually whittling it down to none. What makes it all the worse is that they are his daughters, and he has just disinherited his youngest daughter Cordelia, in their favour, for her inability to flatter him falsely.
Perhaps JA intended for her readers to remember this scene in Lear while reading the Fanny/John dialogue in Ch. 2, as there are so many parallels, IMO.