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?