I've recently found a farming spot in act 2, any quest after 8. The black soulstone should work but I always do this on Lord of Lies (Belail kill). There's a cave called Cavern of the Betrayer level 1, you need to enter this cave during quest 7, Blood and Sand to retrieve Zoltun Kulle's blood, however I discovered that this cave can be the source of some pretty sweet income as well.
I just did a quick run in here with 157% gold find, it took me 5,5 minutes and I earned about 35k during this time, 60/5,5 = 10,9 which means 381k profit, this number will vary alot depending on your movement speed, pickup radius, gold find (magic find is pretty useless unless doing this on inferno or being really lucky with items), your DPS shouldn't really matter alot but as long as it's above 20k they will die really fast. I do this on hell difficulty but it can probably be done on Inferno or nightmare as well, I just find hell most profitable and really easy to do as there is no need to really focus and kite the mobs you kill, I did this as a demon hunter but I am pretty sure you can do this on any class...
Step by Step to do this quest:
- Change act to act 2 on any difficulty, I prefer Hell, then select any quest after 8, I prefer 10. Lord of Lies.
- Choose your follower with highest gold find, I prefer templar as it heals me and keep the enemies at bay for a short time as a demon hunter.
- Choose the waypoint to Desolate sands.
- When you get to Desolate sands, run around and search for the "Cave of the Betrayer 1"
- Enter the Cave and clear whole level 1, shoot all rocks, dead worms/corpses, egg sacks, open any dead bodies, kill all enemies, but you should try to find level 2, meaning if you run to the entrance to level two and realise you have missed a room, do not run back, just enter level 2 as it's a waste of time to go back.
- Do the same on level 2, when you get to the end of the dungeon, just exit the game and resume, you will be back at the beginning of level 1.
- Repeat.
This is the cave, but of course the spawn location is different for everyone: