Regarding the high cost. Lok, where did you get those numbers? Did you play version 1.0 too much? ;) In version 3.0, building tier 7 creatures isn't that expensive. The most expensive ones are the Chaos Druids; they cost around 14,000, I think. But overall, the idea of focusing on gold is great. I even suggest doing it differently, eliminating the resource warehouse altogether and making a building that generates 1,000 gold per day in the economic branch, after the magistrate, as a mandatory building for the capital, not requiring gold for its construction but requiring a significant amount of resources. If the focus is on gold, then the prices will have to be adjusted. You could also create a building that generates 1 unit of ore per day; it would require Kobolds to build. Then, the prices could be increased, and the resource costs could be reduced. If there is racial magic, then Illithids would be built from buildings of that guild; if not, then from a regular guild. I also suggest linking the creatures to a specific level, for example, the Illithids. The city building branch needs to be made as unique as possible; it would be more interesting :)
Bow down and hear our cry,
Our earthly rulers falter,
Our people drift and die,
The walls of gold entomb us,
The swords of scorn divide,
Take not thy thunder from us,
Take away our pride.
G.K. Chesterton: English Hymnal