] Possibly, but that "What might be called economy" suggests to me rather that Elizabeth's fund of spending money was so large that all she had to do in order to be "economical" was simply not go spending-crazy.

Yes, I assumed that, too. I would also like to think that Mr Darcy would have thought ahead somewhat and assumed that Lizzy would want to help out her family in this way. By giving her lots of pin money, which traditionally, he wasn't suppposed to control, he would probably expect her to look after Lydia, and at at the same time he wouldn't have to give any money to Wickham which Lydia would be unlikely to benefit from!

(Isn't that just neat of JA to tie up loose ends like this?)