At first - wars NOT cheap. It costly enough. It is easy to determing reasonable proportion from broken fort price - for example those 1/3 or 1/4 that needed to repair it (even halved like the price of killed army). Than we have not gain income for unrepaired forts - like in the war score we gain points for forts. I'm waiting for others contra...
Tough idea don't sound bad when i think twice i don't think it's good.
In old realms most of the wars were fought with hitting empty forts. We had one war where we won 5k points by JUST hitting empty forts.
Enemy did army save like clock work and we had no other option than to win points in DAMN HARD way.
Usually my experience with wars is that when 2 good alliance fight if one army dies in 7 days you are luck. Most of army killed are archers, some forgoten army, pillages and braking empty forts.
So if they implement your suggestion stone price would go rock bottom.
Imagine you get MILLIONS of stone as war income. What a hell you would do with it.