Too bad you can't rent out homes...I mean, you can own one per hold, so why not rent them out. AND they get a free housecarl for protection! $_$
Though, I have no idea what you would spend the extra income on to be honest. >.<
Lusty argonian maids, skooma, and bards. Or more real estate to become a mogul, I guess.
Mages college is a great place to set up your base if you need dragon bits for armor or soul collecting. I use my old dorm wardrobe for dragon mat storage, and J'zargo apparently hangs out there when he's not with me.
If your mage is a necromancer, Markarth is really nice. You get a pretty sweet house with storage and all that, but then you can use the "House of Horrors" to store your thrall corpses, it puts the evidence away from your home and it's pretty accessible. >:3