The insect Queen is the best source of money with 0 effort if you don't wanna bother with the brothel... though it requires an investment.
Hire a guard from her and level her up, a couple of days later the Queen will meet you and opens up new quests for you. Set up 2 insect girls to breed, put eggs in an "energizing" modded hatchery, and each new girl can immediately be given to the Queen for a sweet 2k gold (edit: 950 gold for a MaxLVL 7 insect girl, and you can also trade each girl for a random legendary item, very good to equip yourself early!!). That, or reduce your rivals' power by 3 each - per girl!
On my current file i get 6 eggs/day, hatching in 1 day, so 12kgold/day OR-18 rivals power/day...
Queen of mid-game, baby. Too bad the portal hostility doesn't decrease like rival power, so you get a hardcore portal and baby-boss fight. Also yeah, the art is amazing.