Once again, I try to reverse engineer the assumptions, because I haven’t learned my lesson.
It looks like most buildings are constructed as a wall with the same circumference and thickness, multiplied by x5 for all of the various design complications and interior walls, stairs, floors, etc.
This works - with some rounding - for pretty much everything except the square keep.
The square keep costs exactly the same as a wall of the same circumference, assuming a continued progression past 60’ height. With the x5 multiplier, it would cost 390,000 gold (or, with the same rounding, 375,000 gold).
Is the keep’s cost an errata? Or is there a factor I haven’t considered? Or am I just plain wrong?
The stronghold construction costs in ACKS are based on the stronghold construction costs found in David Cook's Expert Rulebook on page X52. Those are, I believe, in some capacity, cribbed from either Gygax or Arneson's even earlier work.
ACKS has an implicit conversion rate of 1 silver piece = 1 silver penny (so 1.2gp per shilling, and 24gp per Lb). I used this conversion rate to price out about a dozen historical castles using ACKS and found they worked out to within 25% of the known historical prices, so I left them largely untouched.
I therefore wouldn't overly concern yourself with this. The "square keep" is likely the simplest form of building a castle-builder will undertake, and having it be cheap ends up leading to correctly-priced historical castles overall. The wall prices allow you to have walls meeting at wierd angles and twisting in different directions, so it would make sense that you're "getting a discount".