Cost of walls thinner or thicker than 10 feet?

I'm working on the ACKS conversion of Dwimmermount's fortress of Muntsburg, where the outer walls are 5' thick. How does making a wall thicker or thinner change the stronghold construction costs on p. 126?

 

If the outer walls are only 5' thick, the cost per 100' length is halved.

 

hahaha, how simple and i just have made some excel calculations, calculating the price per cubic feet for existing walls and applying the results to 5’ thickness…at least, my calculations are the same as alex’s

quick napkin math says 4gp/cubic foot of stone?

Yep, but only first two entries:
Hight Length Thickness Cost Volume Cost per cubic-foot
20 100 10 5000 20000 4
30 100 10 7500 30000 4
40 100 10 12500 40000 3,2
50 100 10 17500 50000 2,86
60 100 10 22500 60000 2,67

so we have for 5’ thickness using cost per cubic-foot:
20 100 5 2500 10000 4
30 100 5 3750 15000 4
40 100 5 6250 20000 3,2
50 100 5 8741 25000 2,86
60 100 5 11236 30000 2,67

and 15’:
20 100 15 7500 30000 4
30 100 15 11250 45000 4
40 100 15 18750 60000 3,2
50 100 15 26224 75000 2,86
60 100 15 33708 90000 2,67

Actually I suspect that the cost wouldn’t be decreased that much, since the 10’ thick castle wall is not generally solid stone, but rather thick masonry stone facings with a rubble and dirt infill. A 5’ thick wall is more likely to be your standard solid stone wall (although it is also likely to have some degree of infill. So at a rough WAG I’d say the cost would actually be around 70% of that of the standard 10’ thick wall.

You are correct from a true simulation perspective. For game purposes, it's easier to just treat the thickness of a wall as a proxy for its strength as stone, and manage the cost accordingly...