There is no evidence (that I'm aware of) from the comic for this conjecture, but might the cost of Unit promotion have at least a partial dependency on the current Level of the Unit being promoted? Perhaps a higher Leveled Unit would cost more (or less) to promote, above and beyond the 'Nobility' modifier. I don't have anything to back up that hypothesis, and I don't remember the Level of Wanda (or Tramennis, for that matter) upon the point of Promotion.
It might also have to do with the size of the side the heir is going to inherit. A huge side like Jetstone might have to fork over more money for such a promotion than one like Goodminton. And, Goodminton has been known to raze cities for money:
The Erfworld Wiki wrote:
All Cities save the Capital were lost to Haffaton or razed for Shmuckers following the Kiloton debacle.
Therefore, Overlord Firebaugh might have pulled the "wipe everything that's left for shmuckers to promote Wanda" gambit, in the end.