What we should have is technology research --> better food production --> bigger cities --> city populations pay more taxes (gildar).
Gold mining would be a parallel source of income, with quest loot towards the bottom. I would drastically reduce the monster spawning and eliminate the gold rewarded from everything but believable opponents like bandits. Bandits probably have money. Spiders shouldn't.
The problem with this is you'd have to alter the whole item store price scheme & trade item price perception. If you're going to reduce the gold gotten from monsters then the prices at the store need to be changed as well: 300 gold for amulet of life, 195 for ceder crossbow, 100 for great helm, 2700 for kwarazami (whatever the hell it's called) magic sword. These are all way over-priced.
I disagree with the initial precept that the economic system needs to be "fixed". It works as-is. Sure, in the big picture, 100 acres of crop or gold mine are going to produce a whole lot more income than some ivory tusks, but also the items in the shop would not be so expensive in relation. Frankly, I don't really see a need to rationalize a fantasy type game' economic system; you can rationalize that the skin of troll or giant spider venom is "worth its weight in gold" hence justifying the large loot/booty gotten from "monster harvesting".
In my opinion, scaling the game to get more income from mines (especially when it's hit and miss as to if and when you can find them) would more than likely unhinge the AI tremendously, even more than it is now. Imagine the AI that can't find a gold mine .. iow, it'll just create unecessary additional problems just for the sake of seeming "realistic" which frankly is not worth overhauling the economic system for that reason. Ultimately, it only matters that it "works", and the economic system does in fact work. This is not what needs to be fixed in this game.
My experience has been this: all that additonal cash I get from "monster harvesting" just gets spent outfitting champions. You can spend 2000 gold easily outfitting one single guy with all the armor, amulets, medical pack, longbow, etc .. also the other AI kingdoms ovecharge for their goods, I've not been able to buy 100 crystal for less than 3000 from neighboring AI, again justifying the need for lots of extra cash from "monster harvesting". The only thing I don't like about it is that it gets tedious turn-after-turn fighting the same type battles over-and-over.