I got most of my cash through doing as many quests ad possible and selling the loot off of corpses, but in one party where I had Haer'Dalis, I could make a pretty good amount of cash in a day by having another party member cast 'improved invisibility' on him, occassionally making him gulp down a potion of master thievery, and then pickpocketing people. It does make it easier for him to filch without getting caught if he's invisible.
Minor Spoiler here:
If you have a thief in your party with lockpicking and finding trap abilities, and are doing the Mae'Var quest, the second floor of Mae'Var's guild hall has a lot of doors lined up along the walls of a room. Pick those locks and there's some pretty good loot- the loot gets better as the locks get harder. The *trapped* doors start towards the end of the room where the wall safes start, be sure you're looking for them, because they're fairly nasty. Best part is, you don't get in trouble for picking those locks and looting, even with the NPC shadow thief standing right there watching. It's not really a *fortune*, but if you just need a bit o' extra cash to buy something pretty at the Adventurer's Mart, it's worth checking into.
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Playing the cutest li'l Chaotic characters since 1996.