Empty LEGO boxes: are they worth anything?
On Bricklink.com vendors sell original boxes. You can see a listing of what's available here. For example, the King's Mountain Fortress from 1990 is offered for sale for about $65 USD. (I have the front and back flaps of that box, I wonder if I could list just those.)

Bricklink shows a price guide for products which have been sold recently; Some of the boxes show sales and some of them don't.  Whether you can get any specific price for a box will depend greatly on the condition of the box, how you manage your bricklink reputation, and how patient you are.