There's no pat answer, as it's entirely dependent on the gun, and the extent of the damage. It's amazing what a good stockmaker can repair.
To simplify it a bit though, let's assume only a restock will cure it's ills. An otherwise nice English boxlock that would be worth, say, $3500 with good wood, but in need of a complete restock, would be a push if the gun were free, assuming the work is done to original quality standard. A Holland Royal in .500/.465 in otherwise excellent condition (a beast to restock due to the Baker extended top tang), wouldn't require more than a 20% discount.
Keep in mind that a nice double gun freshly restocked in a manner not commensurate with original quality standard is just another buggered double gun that still needs to be restocked, and the market will treat it accordingly.
