Miasmas have good drops but IMHO, if you actually calculated how much you make per second, they aren't as great as you imagine because of how long it takes to kill them. The fact that a single disconnect or lag spike could get you killed very quickly and easily and cost you a lot of insurance money makes it even chancier.
If you really want to make gold, you don't go for the biggest, toughest, hardest things that drop the most gold. Go for the things that can be killed the quickest, and most consistently, which you can sustain fighting over a long period. You might even actually make more money killing things like earth and air and water ele's in the first few levels of Shame because of your speed and ability to keep going quickly, rather than killing 1 miasma taking 1 minute and 30 seconds or however long it takes ya. (Plus the added time running or invis'ing when it gets mad at you.)
That's just as far as monster hunting goes. While you can make a lot of money monster hunting, I do not consider it necessarily the best way, at least, not unconditionally so. Especially as a mage... you need mana, you need range, you need downtime/meditation time. I think you might be amazed (I just GM'med inscription recently) how much money you can make doing something simple like like spending the same amount of time scribing as hunting good spawn. In a half hour of making something non-mana intensive like 4th circle scrolls and selling them back just to the NPC's.... I make many,many thousands. At no risk of death and no running around or invis'ing or competing with other players for monster respawns.
The best strategy, is probably to incorporate a little of everything. When I am in town or in my house, I am almost ALWAYS scribing, at the very least I prevent my mana bar from ever getting full. I now consider it a waste of time when I see so many other players just sitting around town doing.... nothing. With five to ten minutes of idle time in town I can earn thousands of gold just scribbling on some paper.