Augh. How is everyone getting so many septims? I would say that I only have about 1000-200 pieces currently. I'm finding it to be really hard to get rich. :/
All I can do is make and sell potions...
Well, in my case, the merchants are modded to have a lot more gold and at high Mercantile no longer rip me off, so I get full value for almost everything. As a result, at high levels I can get from 150-250K from one fort or ruin, and about 80-100K from caves. The largest Marauder forts can net me up to 300K, although anything over about 270 is fairly rare. Oblivion gates also pay in the 200K+ range, although their extreme size compared to spawn density tends to make them a slog.
That does depend on the inhabitants; critter dungeons usually yield far less loot than NPC ones, and among the NPCs Conjurers or Necromancers typically yield less than Bandits or Marauders.