When we saw the trade routes, my group just assumed it was a typo (-LM instead of +LM) in the formula. Basically the same idea you have.
For kingdoms, the buildings are constructed in 1 month. I'm guessing the increased cost is for getting it done that fast. The simpler buildings that could actually be built in that time with the room construction rules seem to have a much smaller 'price gap' with the kingdom rules.
