Not sure if this is wildly known information by now, but I feel confident that I've found the best way to make money once you reach the mid game.
Buy any number of equippable items, and a bunch of essence. Enchant all of the items with the enslavement property. Then go to one of the "very dangerous" spots in the city, and explore.
If you beat Succubus you encounter (usually around level 7 or so), you can equip them with the enslavement item, and sell them for around 50k. It isn't hard to generate a million gold within the span of 20 minutes or so.
This is a massively better money making method than the other one I saw people listing (which is enchanting food and selling it to the snack vendor).
Since milking was nerfed, the best way of making money (and getting arcane essences and leveling up in the process) are cultists, they suppose near no threat and give you an expensive set of witch clothes and broom. They also drop Lilith's Gifts and some money. Just spam '1' and the spacebar.
For the best results you can put your difficulty at Lilith and spawn an elemental (The elementals are not affected by the damage reduction of this difficulty)
In a minute: I defeated the same cultist 76 times, made around 190,000 raw flames, made about 2,800,000 flames from selling drops, got around 4300 essences (not even on an arcane storm), leveled up an npc from level 1 to 50. And I wasn't even being as effective as I could, as later I realized my elemental was sometimes using "Soothing Waters" instead of attacking.
Rarely does any text porn game have such a variety of possibilities. But there is not enough depth and elaboration, which is sad, of course. And judging by what I read here on the forum, the author is not particularly engaged in this, unfortunately 
