I'm not sure why nobody has mentioned this; perhaps everyone is opposed to cheating... but some answers have suggested cheating, so here I will suggest one.
You could duplicate some things. I'm not sure if you know about this (I don't know how well-known it is; pretty well-known, I assume). Are you playing for xbox or PC?
Anyway, if you're playing for x360, AND you have the Shivering Isles expansion... it's very easy to make money. And get 100 Alchemy skill.
Ok, first off, I'd suggest getting 100 scrolls (the SAME scroll; i.e. 100 Beguile scrolls). It doesn't matter what scroll it is, as long as you have a sizeable amount of the same scroll. Ok, equip the scroll (in your INVENTORY tab, NOT your Spells tab; just highlight it and click A). After it's equipped, drop any item you want. You'd probably want to drop something that's worth a lot, i.e. a Strong potion of some sort, or even weapons or armor. Now exit your inventory/journal menu thing. Whatever item you dropped will be dropped in the amount of scrolls you had selected. Example: If you had 100 Beguile scrolls and dropped 1 sapphire, 100 sapphires will be dropped. This works for pretty much everything, excepting a few different ingredients, quest items, and sometimes weapons. I couldn't do it with my Mages' Staff that I got from the Mages' Guild, but I could do it with the Apotheosis staff I got from Rindir's Staffs in the Imperial City market district. (Not only does that staff do 99 damage, it's also worth around 3,000 septims, I believe. Depending on your Mercantile skill, of course.)
Note: I would suggest not trying to drop over 200 items. 150 or 100 would be good. Typically any more than that makes the game really choppy and prone to freezing up on you. Also, if you drop an item and it doesn't duplicate it, try it again. Sometimes you have to try it twice before it'll work (this generally happened to me if I tried to duplicate an item I had bought and hadn't actually "picked up" in the game beforehand).
So, if you're not opposed to cheating (if you consider this cheating, which I suppose it is), I think this is the easiest way to make money. I do it so I can start the game out with a house and horse and such. It makes the game more fun for me, so that justifies it in my mind.
